Delay line

Localized and stationary dynamic Brillouin gratings using PRBS modulated pumps are applied to delay periodic, 0.5 ns-long pulses by up to 1 µs. To realize all optical delay line for pulses the setup shown in the Figure 42 was built. A single distributed feed-back (DFB) laser diode emitting at frequency ω was used to generate both pump waves. The output of the ₁ DFB was modulated by an electro-optic phase modulator (EOM), which was driven by a PRBS generator. The output peak-to-peak voltage of the PRBS generator was adjusted to match V_π of the EOM (~3.7 V). The bit rate of the PRBS generator was controlled by an external microwave generator. The modulated DFB light was split in two arms. Light in the upper arm was amplified by Er-doped fiber amplifier (EDFA) up to 200 mW, polarized by polarization beam combiner and launched in the PM fiber along the slow axis. Light in the other arm was intensity modulated by another EOM 2 with frequency Ω , where the bias of _B

the modulator was adjusted to suppress the carrier. By using a narrow FBG the upper sideband at frequency

ω

₂ = ω + Ω was carefully filtered, amplified by another EDFA 2 to _{1 B} 200 mW and launched inside the PM fiber along the slow axis from the opposite direction as a pump 2. Light from a distinct DFB 2 was used to generate the signal wave. The optical frequency of the signal DFB was adjusted to match the DBG resonance along the fast axis using equation: 2 2 2 2 ( xp ) p sig g x y n n ω ω =ω + Δ =ω ω +Δ ω (3.88)

Light used as readout signal was pulse modulated using EOM 3 amplified by an EDFA 3 and redirected in the PM fiber along the fast axis. Reflection from the dynamic grating at frequency ω = ω + Δ −Ω was filtered by another FBG, boosted and detected _{ref 2} ω _B using 6 GHz photodetector.

Figure 42. Experimental setup to realize all optical delay line using PRBS phase modulation of the pump waves.

To observe the delay for signal pulse, the position of the DBG was continuously changed from one end to the other end of the 100-m PM fiber through changing the bit duration of the PRBS generator. The Brillouin frequency of the fiber was measured to be 10.87 GHz, while fiber birefringence gives difference in frequencies between pump 2 and signal wave of Δω = 2 × π× 57 GHz. A PRBS code word length was set to 10

2 1

M = − and a fixed delay imbalance was added to the path of pump 1 so that the 10th correlation peak was scanned along the fiber under test. Figure 43 shows seven selected time traces of the reflected pulses from the DBG at different grating positions, where the difference in arrival time between the black pulse and the blue pulse is 770 ns. A variable delay of 1 µs can be easily obtained which corresponds to the round trip time of 100 m fiber. The most right peak in Figure 43 corresponds to the parasitic reflection from connector, which was not filtered out.

Figure 43. Detected pulse reflection from the DBG, localized at seven different positions along the fiber. Black dashed lines correspond to the beginning and the end of the fiber. The PRBS modulation clock rates 1/T were (left to right):

1.12 GHz, 1.108 GHz, 1.093 GHz, 1.078 GHz, 1.063 GHz, 1.048 GHz, 1.033 GHz.

Initial and reflected pulses are represented in Figure 44 (a). It is clear that broadening is negligible in this case and as before it is governed by the length of the DBG. Slight distortion on the leading and trailing edges of the reflected pulse can be attributed to filtering effect of DBG. Figure 44 (b) demonstrates the fine tuning of the delay by sub-ns step. Delays (from left to right) in the Figure 44 (b) were obtained by changing the clock rate of the PRBS modulator with the increment of 100 kHz. It should be pointed out that position of the correlation peak can be controlled with extremely high accuracy. Accuracy depends on a precision of the microwave generator driving the PRBS as well as on a correlation peak number.

Figure 44. a) reflected (magenta curve) and initial pulse (blue curve). b) Selected waveforms of the reflected pulse demonstrating delays with sub-ns steps.

Bandwidth of the DBG gratings generated by the PRBS phase modulated pump waves was investigated using a Vector Network Analyzer (VNA). An EOM in the signal arm (see Figure 42) was driven by the output port of the VNA, and the detected reflection was analyzed using the input port of instrument. Figure 45 shows half of the bandwidth of the DBG generated by the phase modulated pumps, using PRBS with different clock rates. In Figure 45 the x-axis represents the offset frequency from ω , which is at the center of the _sig DBG resonance. It is clear that dynamic grating bandwidth becomes larger with increasing the PRBS clock rate (reducing the bit length), since the grating becomes shorter. Moreover the DBGs bandwidth is in the order of PRBS clock rate. This can be used to estimate the maximum bandwidth of the signal, which can be delayed without severe distortion. High reflection at the low frequency offset (<200 MHz) is due to the residual long grating, spanning over the entire PM fiber. Such residual grating can be removed if the driving voltage on the phase modulator match precisely V_π.

Figure 45. Measurements of the DBG reflection bandwidth using vector network analyzer. PRBS clock rates were: 1 GHz, 2 GHz, 4 GHz, 6 GHz, 11 GHz.

In document Generation and application of dynamic gratings in optical fibers using stimulated Brillouin scattering (Page 114-117)