Cascaded Long Period Fibre Grating-Based DPSK ...

2 downloads 0 Views 92KB Size Report
Tae-Young Kim (1), Masanori Hanawa (2), Youngjoo Chung (1), Won-Taek Han (1), Chang-Soo Park (1). 1 : Gwangju Institute of Science and Technology ...
Cascaded Long Period Fibre Grating-Based DPSK Demodulator with Optically Tunable Phase Shifter Tae-Young Kim (1), Masanori Hanawa (2), Youngjoo Chung (1), Won-Taek Han (1), Chang-Soo Park (1) 1 : Gwangju Institute of Science and Technology (GIST), 1 Oryong-dong, Buk-gu, Gwangju, 500-712, South Korea Phone: 82-62-970-2218, FAX: 82-62-970-3151 Email: [email protected] 2 : Univ. of Yamanashi, 4-3-11 Takeda, Kofu, Yamanashi 400-8511, Japan, Abstract A DPSK demodulator based on cascaded long period fibre grating is proposed. The performance of the demodulator is demonstrated for a 10-Gb/s DPSK system, showing clear eye diagram as well as bit error free performance. Introduction SMF YDF SMF Cladding Optical differential phase shift keying (DPSK) has recently attracted much attention as a suitable modulation format for the high-speed transmission L Core LPG1 LPG2 system [1]. The optical DPSK system requires a L Cladding mode DPSK demodulator at the receiver side to convert the 0 π 0 0 0 π π 0 phase modulated signal into the intensity modulated Core mode π 0 π π π 0 0 π signal for direct detection. Most of the DPSK Demodulated signal demodulators were implemented based on MachZehnder interferometer (MZI) with a time delay in one Fig. 1. Working principle of the demodulator based arm. However, the interferometric structure of the MZI on cascaded long period fibre grating. is sensitive to environmental fluctuations, thus modes, the core mode and cladding mode. They have demanding an additional stabilization circuit. Recently, different propagation velocities due to the index we reported a cost-effective DPSK demodulator difference, that is, the cladding mode is faster than based on of the phase shifted fibre Bragg grating the core mode. At the second LPG (LPG2), they are (PSFBG) of a reflection type for relaxing the instability combined and split again into two; therefore this of the MZI [2]. The PSFBG is insensitive to the CLPG acts as an optical delay interferometer with the environmental perturbation because it has a shared precise time delay. lightwave path. However, the PSFBG has a large The time delay is given by the following equation. insertion loss due to its low reflectivity to suppress 1

2

multiple reflections between two gratings. In this paper, we propose a DPSK demodulator based on the cascaded long period fibre grating (CLPG) with an optically tunable phase shifter using an Yb3+ doped optical fibre (YDF). Because the CLPG provides the MZI structure of the transmission type, the multiple reflections is free differently from the FBG, reducing the insertion loss. The time delay between two modes is short from small difference of reflective indices, and thus is suitable for the high speed DPSK system more than 40-Gb/s. By using the phase shifter inserted in the CLPG, moreover, the phase difference of the lights in the core and cladding regions can exactly and fast be controlled [3]. Principle and Experiment The working principle of the proposed demodulator is shown in Fig. 1. It consists of two LPGs with the distance of L1 and the piece of the YDF. Between the two LPGs, the YDF of length L2 is inserted. Although the distance of two LPGs could be relatively long compared to the PSFBG-based demodulator, it gives an advantage of spatial margin to accurately implement a short time delay. When the DPSK signal propagating in the core mode meets the first LPG (LPG1), then the signal is split into two propagation

ΔT =

(L1 − L2 )Δmeff

SMF

+ L2 ΔmeffYDF ,

c

where L1 is the distance between the LPGs, L2 is the length of the YDF. c is the speed of the light in the vacuum, and ΔmeffSMF and ΔmeffYDF are the difference between the effective group indices of the two modes in the SMF and YDF, respectively. The phase difference of the core and cladding mode should be π to destructively demodulate the DPSK signal. At the particular wavelength, however, it is difficult to make the exact phase difference of two modes. Thus, a certain length of the YDF is inserted between two LPGs to realize the optically tunable phase shifter. When a pumping light from a laser diode (LD) with the centre wavelength of near 980 nm is launched into the YDF, the refractive index of the core of the YDF is changed, resulting in the phase change only in the core mode. Thus, the phase difference of two modes can be optically controlled. The trial demodulator with the time delay of 25 ps was fabricated for the proof-of-principle experiment. Basically, the time delay of 100 ps is required for the 10-Gb/s DPSK in the non-return-to-zero (NRZ) format demodulation. However, because the length of the

PPG Clock Trigger

WDM

WDM

YDF LPG

Pump LD Error Detector Scope

O/E

VOA

Fig. 2. Experimental setup. TLS: tunable laser source, WDM: 980/1550 WDM coupler, LPG: long 3+ period fibre grating, YDF: Yb doped optical fibre, VOA: variable optical attenuator. demodulator became so long for the time delay of 100 ps, we chose the 25 ps delay, getting the 10-Gb/s OOK signal in return-to-zero (RZ) format demodulated from the 10-Gb/s DPSK in the NRZ format [4]. The L1 and L2 of the fabricated demodulator were 61 cm and 50 cm, respectively. Fig. 2 shows the experiment setup, in which the tunable laser source was modulated with a 10-Gb/s pseudorandom binary sequence of 231-1 using a phase modulator. The phase-modulated signal was sent to the DPSK demodulator. The pumping light, used for the phase control, had a centre wavelength at 976 nm, and was launched into the YDF through a 980/1550-nm WDM coupler.

0 mW 15.1 mW 28.3 mW

0

LPG

-5

-10

-15

0.32 nm

5

Phase shift (radian)

Data

PC

4 3 2 1 0

-20 1544.5

1545.0

0

Wavelength (nm)

10

20

30

40

50

Pumping power (mW)

(a) (b) Fig. 3. (a) Optical spectra measured at the pumping power of 0, 15.1, and 28.3 mW, (b) pump-induced phase change calculated from the wavelength shift.

4 5

-log10 (BER)

EDFA Phase Modulator

Transmission (dB)

PC TLS

6 7 8 9 10 11 12 13 -21

-20

-19

-18

-17

-16

-15

-14

Received optical power (dBm)

Results and Discussion The optical spectra of the fabricated demodulator, shown in Fig. 3, were measured at the pumping power of 0, 15.1, and 28.3 mW. The interference fringe pattern was observed with the free spectral range (FSR) of 0.32 nm (40 GHz). As the pumping power increased, the fringe moved to the longer wavelength. This is the clear evidence of the phase change between two modes, obviously attributing to the index change of the core in the YDF due to the launched pumping light. The amount of the phase change was calculated from the wavelength shift in the fringe pattern, plotted with pumping power as shown in Fig. 3(b). The amount of the phase shift increased linearly as the pumping power increased. To obtain π phase shift, the pumping power of ~28 mW was required. The eye-diagram was measured using a sampling scope with 20 GHz bandwidth as shown in the inset of Fig. 4. As previously mentioned, the eye-diagram was a return-to-zero (RZ) shape because time delay of the demodulator was shorter than the 1-bit duration, and was clearly opened. The demodulating performance for the 10-Gb/s DPSK signal was assessed with the bit error rate (BER) measurement as shown in Fig. 4. We could observe the error free performance and the receiver sensitivity at a BER=109 was -17.6 dBm. The disadvantages of the proposed demodulator are that the total device length is long due to the small difference between the effective group indices of the two modes and the cladding mode is very sensitive to the bending and contact. These disadvantages can

Fig. 4. Measured bit error rate of the demodulated 10-Gb/s DPSK signal (inset: corresponding eye diagram). be overcome by using the few mode fibre, which can be winded [5]. Conclusions We have proposed the DPSK demodulator based on the CLPG and the YDF in which the phase of the DPSK signal in the core of the YDF was optically controlled. The performance of the proposed demodulator was successfully demonstrated for the 10-Gb/s DPSK system, showing clear eye diagram as well as bit error free performance. The phase controllability for the core mode was experimentally demonstrated, showing that the phase was linearly shifted by controlling the pumping power at 976 nm. Acknowledgement This work is partially supported by KOSEF through grant No. R01-2006-000-11088-0. References 1 Gnauck, A. H. et al, J. Lightw. Technol., 2005, 23(1), pp. 115–130 2 Kim, T.-Y., et al, IEEE Photon. Technol. Lett., 2006, 18(17), pp. 1834- 1836 3 Kim, Y., et al, Opt. Express, 2004, 12(4), pp. 651656 4 Peucheret, C., et al, IEEE Photon. Technol. Lett., 2006, 18(12), pp. 1392- 1394 5 Ramachandran, S, et al, IEEE Photon. Technol. Lett., 2001, 13(6), pp. 632- 634