Dual Pump Configuration on Oscillator of Wavelength Conversion on Four Wave Mixing Using PCF

Mohd Nizam Abdullah1,2, Sahbudin Shaari1, Abang Annuar Ehsan1, Mohd Nasir Zainal Abidin2,

Abdul Rashid Zainal Abidin2 1Institute of Microengineering and Nanoelectronic

Universiti Kebangsaan Malaysia 43600 Bangi, Selangor, MALAYSIA

2National Metrology Laboratory, SIRIM Berhad, Lot PT 4803, Bandar Baru, Salak Tinggi 43900 Sepang, Selangor, MALAYSIA.

ABSTRACT

The work presented here shows the effect of wavelength conversion experiment on Four Wave Mixing (FWM) by utilising photonic crystal fibre(PCF). This configuration consists of dual pump and a few sets of Fiber Bragg Gratings (FBGs). Selective conversion is also possible by adjusting on one of pump laser. However, phase matching conditions plays vital roles in this experiment. This phenomenon is the major cause of FWM. Keywords: Erbium doped fibre(EDF), fibre Bragg grating(FBG), photonic crystal fibre(PCF), four wave mixing (FWM)

1. INTRODUCTION One of non linear effects is FWM which gives an impact towards telecommunications industry either as advantage or disadvantage from its consequence. It also plays an important role in advanced multi channel light wave systems. Currently, optical multi wavelength converter such as incoherent light in a dispersion-shifted fibre [1] and multimode FBGs [2] are among the method in production of FWM as wavelength conversion. As an addition, FWM optical fibre is also one of the methods in wavelength conversion [3]. As for our approach, optical wavelength conversion and selection is being experimental studied in fibre ring laser with inclusion of photonic crystal fibre. This paper describes experimental works done using PCF and non linear effect such as FWM is the prime objective study of this experiment on its effect towards wavelength conversion and selective among the wavelengths involved in enclosure cavity. This technique approach was totally defiant from the experimental works done previously. The main component of this experimental study in production of FWM is PCF which has a zero dispersion of 1060nm. Even though it characteristics is more suitable for 1060nm range lasers in efficiency of non linear effects but we intend to investigate FWM existence beyond zero dispersion experimentally. As per our knowledge most experiments are done within zero dispersion region fibres.

The structure of this paper is organised into four major segments. In segment 1.1, related on FWM development in enclosure cavity due to the effects of FBGs. Experimental configuration and components involved in the experiments is further explained in segment 1.2. Then, in segment 1.3, discussion on the outcome due to experimental results based on the optical spectrum measured by optical spectrum analyser (OSA). Lastly, in segment 1.4, we conclude the findings of the experimental set-up done on multi wavelength conversion and selectivity based on FWM effect.

1.1 Theory

FWM is a third–order nonlinearity where several optical signals at different wavelengths propagate along the fibre [5]. For example, when three optical wavelength (λ1, λ2 and λ3) interact in a nonlinear medium, it will generate a fourth frequency, f4 where;[5]

3214 λλλλ −+= (1)

Photonic Materials, Devices, and Applications III, edited by Ali Serpengüzel, Gonçal Badenes, Giancarlo C. Righini, Proc. of SPIE Vol. 7366, 736623 · © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.821743

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Besides that, interaction of two wavelengths does eventually generate FWM, a sideband at each wavelength, 2λ0 as shown in equation (2). These sidebands travel along with the original waves and develop in the expense of the signal strength input [6]. Therefore, sufficient along the fibre propagation does effects the existence of FWM.

2102 λλλ += (2)

Interactions of multiple wavelengths in the event of EDF lasers had developed the interest to investigate FWM in fibre ring laser mode. A dual pump configuration is applied to accomplish wavelength conversion and selective process in an oscillator. Two main factors which influence this event are phase matching and non degenerate FWM. Based on our experimental set-up, phase matching characteristic is far-off from zero dispersion wavelength was experimentally investigated. Whereas, a selective wavelength process which occurs during the oscillation in caused of the FBGs reflections wavelengths shall produce an arbitrary lasing wavelengths regardless of zero dispersion of PCF. The efficiency of FWM can be derived from power variations equations (3) and (4), [3]

01

100 2 P

PP η

ληλ

−=Δ (3)

10

011 2 P

PP η

ληλ

−=Δ (4)

where P0 and P1 are the powers of λ0 and λ1 respectively. While λ0 and λ1 are wavelengths lasing by the FBGs and signal from TLS and η is the efficiency of FWM process. The efficiency of this process can derived from equation (2) and (3) as shown in (4). This equation only applied during the interactions of two wavelengths.

( )

⎟⎟⎠

⎞⎜⎜⎝

⎛⎥⎦

⎤⎢⎣

⎡−+−

−Δ=

010

01

1

10

10

22 PPPP

PP

λλ

λλ

η (5)

1.2 Experiment

The experimental set-up consists of fibre ring laser set-up with the inclusion of dual pump light and a signal generated from a tunable laser source (TLS) as shown in Figure 1. A 11m erbium doped fibre (EDF) was used as part of amplifier set-up with a combination of dual pump 980nm source, a TLS, optical isolator and 980/1550nm WDM coupler. In the enclosure cavity, a circulator and a set of three FBGs with different characteristics is well positioned. These sets of FBGs; FBG1, FBG2, FBG3, have a central wavelengths of 1530.47nm (λ1), 1561.42nm (λ2) and 1563.95nm (λ3) and reflectivity’s of 89.9%, 91.6% and 96.7% respectively. The presents of variable optical attenuator (VOA) which function to adjust the reflection spectrum of FBGs are positioned side by side of the FBGs. Simultaneously, when applied its generate the wavelength reflection of FBGs and EDF effects tremendously as a homogeneous gain medium at room temperature [3]. For example, if FBG3 is able to reflect more power compared to the other FBGs, λ3 lases while λ1 and λ2 are suppressed. The details are shown in results and conclusion paragraph.

A 20 m long PCF was located external of the oscillation cavity. This single mode nonlinear PCF which has the characteristics of 11W/km nonlinear coefficient, attenuation of less than 1.5dB/km and mode field of diameter is 4.0µm.

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The main substance of this PCF is pure silica with cladding diameter of 125µm, coating diameter of 244µm and coated with Acrylate. The measurements spectrum output are measured by OSA with a resolution of 1.0nm.

Fig.1. Experimental set-up of wavelength conversion and selective conversion in effect of FWM

1.3 Discussion

The experimental results illustrate FWM by utilising PCF which positioned outer of the oscillator. The output from Pump 1 and Pump 2 was set at 121.49mW and 201.61mW respectively and co-directional pumping with signal from TLS. The oscillator operates multi wavelength lasing is determined by forward pumped EDF gain spectrum. The signal was positioned at 1550nm and an output of -5dBm was set. In Fig.2, exhibits the spectrum generated through this arrangements. Meanwhile, Fig.3 displays the same experimental set-up but the signal output was set at -10dBm. Multi-wavelength lasing generated in this oscillation with a constant spacing due to FBGs and assisted by FWM process which annihilates photons from the lasing to create new photons at different wavelengths. This continuous effect does stimulate EDFA and amplified the process. Subsequently, the lasing wavelengths represent the effects of each FBGs and the non linear effect of FWM. In this situation, sufficient signal power for the FWM process to generates additional lasing. Launching different signal power towards the cavity, the FWM effect generates dissimilar lasing wavelengths. It shows that, signal power level does stimulate FWM in wavelengths lasing of this experimental oscillator. Both experiments demonstrate different conversion wavelengths as stated respectively in Fig.2 and Fig.3.Based on fig 6-8, it shows the relativity of laser power effects towards FWM. The experimental settings for these experiments are based on the laser current supply; the increase of the current shall produce higher laser output. These figure does shows the increment of current supply and FWM effects respectively. This effect is laser power dependent and visibly produced with incremental power laser input towards the oscillation.

We also demonstrate selective lasing due to the same experimental set-up as wavelength conversion. In this experiment, VOAs plays a vital role to demonstrate selective lasing wavelengths produce by experimental oscillator and the stability of each lasing was due to PCF presence. Each VOA was fine-tuned and the experimental results are shown in Fig.4 and Fig.5 respectively. Selective wavelength lasing was anticipated phase matching characteristics for non degenerate FWM. Fine–tuning on both VOAs has remarkably impact the selectivity process on wavelength lasing and added by homogeneous gain medium by EDF in oscillator. Once the oscillator activated, two lases, λ2 and λ3 are stimulated by two degenerate FWM processes.

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At the same time, the degenerate of FWM leads to energy transformation from λ2 which is higher power to lower power of λ4 by suppressing λ2 and λ3. In the process of two degenerate FWM, for example λ1+ λ3 = 2. λ2 and λ2+ λ1 = 2. λ3, both wavelengths are annihilated to produce another wavelengths. Subsequently, λ2 and λ3 which has higher power than λ1, had undeveloped λ1. Adjacently, lase λ3 amplified more than λ2 because of mode competition due to harmonised gain broadening. Both Fig.4 and Fig 5 clearly shown at different power level of signal from TLS and fine-tuning of VOAs, selectivity of wavelengths occurs. A few wavelengths or sidebands emerged eventhough a FBG was used to develop a new wavelengths as shown in Fig.5.

Fig.2.Output spectrum at 1550nm signal and -5dBm with conversion wavelength at 1585.8nm, 1567.7nm, 1583.9nm and 1588.6nm.

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Fig.3.Output spectrum at 1550nm signal and -10dBm with conversion wavelength at 1581.4nm, 1586.8nm, 1591.6nm and

1584.5nm.

Fig.4.Output spectrum at 1550nm signal and -10dBm with selective lasing at 1563.8nm, 1549.9nm, 1530.7nm, 1584.8nm,

1586.4nm, 1590.6nm, 1539.6nm and 1512.9nm.

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Fig.5. Output spectrum at 1550nm signal and -5dBm with selective lasing at 1563.9nm, 1550.1nm, 1531.8nm, 1596.3nm,

1581.0nm and 1587.9nm.

Fig.6. Output spectrum of laser pump current at 450mA, 1550nm signal and -20dBm with selective lasing at 1530.39nm,

1550.0nm, 1561.2nm, 1563.8nm, 1583.0nm, 1581.2nm and 1553.0nm.

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Fig.7. Output spectrum of laser pump current at 400mA, 1550nm signal and -20dBm with selective lasing at 1530.3nm,

1550.0nm, 1561.4nm, 1563.9nm, 1596.8nm, 1572.6nm, 1581.8nm and 1583.7nm.

Fig.8. Output spectrum of laser pump current at 350mA, 1550nm signal and -20dBm with selective lasing at 1530.3nm,

1550.1nm, 1561.5nm, 1564.0nm, 1583.4nm, 1508.1nm, 1571.0nm and 1588.3nm

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1.4 Summary

We had successfully generated FWM through dual pump configuration in enclosure cavity beyond zero dispersion. Wavelength conversion and selective conversion of wavelengths lasing was presented by utilising the effectiveness of PCF and also VOAs. Even though the FWM sidebands are low power level but it’s determine the existence of FWM through this oscillation.

REFERENCES

[1] Four Wave Mixing of Incoherent Light in A Dispersion-Shifted Fiber Using A Spectrum-Sliced Fibre Amplifer Light Source, Y.S Jang, Y.C.Chung IEEE Photonics Technology Letters vol.10 No.2 February 1998

[2] Xinhuan Feng, Hwa-Yaw Tam, P.K.A. Wai Switchable Multiwavelength Erbium Doped Fibre Laser With a Multimode Fibre Bragg Grating and Photonic Crystal Fibre IEEE Photonics Technology Letters vol.18 No.9 May 2006

[3] Switchable and Tuneable Mutiwavelength Erbium Doped Fibre Laser with Fiber Bragg Gratings and Photonic Crystal Fibre, Xeuming Liu, Xiaoqun, Xiufeng Tang, Junhong Ng, Jianzhong Hao, Teck Yoong Chai, Edward Leong Chao Lu IEEE Photonics Technology Letters vol.17 No.8 August 2005

[4] Kyo Inoue, Hiromi Toba Wavelength Conversion Experiment Using Fibre Four Wave Mixing IEEE Photonics Technology Letters vol.4 No.1 January 1992

[5] Gerd Keiser, Optical Fibre Communications, McGraw Hill, pp 498-499 (2000) [6] Mohd Nizam Abdullah, Sahbudin Shaari, Abang Annuar Ehsan Four Wave Mixing in Fibre Ring Laser With

Inclusion of Photonic Crystal Fibre, ICSE 2008 Proc. Paper No.128, pp581-583(2008)

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