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Passive Mode-locked Fiber Ring LaserXUE JIN, Wong Jia Haur and Wu Kan

Abstract—In this paper, simulation of a classical passive mode-locked fiber ring laser model is given first to investigate the variation of the output parameters with different dispersion and nonlinear effects. After it, an experiment based on Carbon Nanotube (CNT) saturable absorber is carried out with a newly-developed device integrating multiple functions. By substantially simplifying the cavity with this component, pulses with a fundamental repetition frequency of 180MHz and temporal duration of 500 femtosecond (5×10-13) are finally generated.

Keywords—Optical fiber lasers, optical pulse generation, ultrafast optics.

I. INTRODUCTION

Among various laser sources, fiber laser is usually meant to be a laser with optical fibers as gain media. These fibers are doped with rare-earth elements and used as gain media for their broad gain bandwidths and excellent beam quality. Fiber laser is directly pumped by a laser diode to create population inversion for the lasing process to take place. With numerous advantages including flexible setup, compact size, large gain bandwidth, etc, fiber lasers are gaining popularity over various bulk solid-state lasers.

In this paper, a system of ultra-fast fiber ring laser with passive mode-locking technique based on Carbon Nanotube Saturable Absorber will be the basis of studies. The objectives are to investigate the laser performances and optimize the results with a newly-developed component called TIWDM which greatly simplifies the entire setup. In addition to obtaining parameters such as pulse width, fundamental frequency, pulse bandwidth, pulse shape and so on, the stability of the passive mode-locked ultra-fast fiber ring laser is also investigated.

II. SIMULATION

Before experiment, a simulation based on Spilt-Step Fourier Method [1] is done first to investigate the variation of output parameters with different dispersion and nonlinear effects in the laser cavity.

As shown in Figure 1 below, a classical simulation model is constructed with a wavelength division multiplexer (WDM), erbium doped fiber (EDF), isolator, carbon nanotube, output coupler and some single mode fibers. For simplicity, we use fiber length as an approximation of total negative dispersion, since single mode fiber is significantly longer than EDF, and saturation power of carbon nanotube as a measure of nonlinear effect. Therefore, different output results can

be obtained simply by adjusting the parameters of fiber length and saturation power in the simulation program.

Fig. 1. Classical model for simulation

In this simulation, we have obtained three forms of data for 3k, 5k and 10k round trips of the pulse development, respectively. More round trips means higher accuracy of the final results, while the trend of the outputs are basically the same.

One typical result is, for example:

Fiber Length: 20m;

Saturation Output Power: 0.1W;

Steady State Pulse Width: 3.00820E-13s;

Steady State Pulse Bandwidth: 8.2038E+11Hz;

Steady State Pulse Peak Power: 87.6773W;

Steady State Pulse Power: 0.7373W;

Then fiber length and saturation output power are changed to other selected values (50 sets in total), and the four output parameters will be obtained correspondingly if another mode-locked state occurs.

Through listing and comparing of the output data, following tendencies or characteristics of the passive mode-locked fiber ring laser were found:

a). Steady state pulse width keeps decreasing with the increasing of saturation power before reaching a minimum value at about 0.2 w for a total fiber length of 20m.

b). Steady state pulse width decreases with the increasing of total fiber length.

c). Steady state output power and pulse peak power increases with the decreasing of total fiber length.

d). Steady state output power and pulse peak power increases with the increasing of saturation power.

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In these findings, a and b are due to the complex interaction of dispersion and nonlinear effects inside the cavity; c and d are simply the results of a reduced total loss in the setup.

Ultra-fast laser system produces short pulses, and the pulse width in time domain is inversely proportional to the bandwidth in frequency domain. Therefore at the same time when pulse width decreases, the bandwidth should increase. To get the desired ultra-short pulse, bandwidth of the mode-locked laser should be maximized.

III. EXPERIMENT

A. EXPERIMENT OBJECTIVEa). To construct a carbon nanotube based passive

mode-locked fiber ring laser.

b). To make repetition frequency as high as possible.

c). To make pulse width as short as possible.

B. BACKGROUND THEOTY Basically, the reference model we adopted was the

ring cavity shown above in Figure 1 as simulated. Ring configuration is preferred because unidirectional directional ring cavity has a fundamental pulse repetition frequency twice that of a Fabry-Perot cavity for the same total fiber length. Also ring configuration is less susceptible to back reflection and has higher output energy.

For different laser configurations, different kinds of pulses will be generated. Generally there are soliton laser, Gaussian laser and similariton laser for respective purposes. Among them, if a cavity is constructed mostly with anomalous dispersion fibers, then it is soliton laser. Besides with saturable absorbers, soliton laser is also compatible with Kerr-type mode-locking technique such as nonlinear polarization rotation (NPR). Here we use saturable absorber as one of the methods to generate solitons. To make soliton laser, one condition must be satisfied: Total cavity dispersion must be negative. In this case, since only single mode fiber and erbium doped fiber are used, the condition can be re-written as:

(1)

where ���is the propagation constant introduced in literature review, LSMF and LEDF are the fiber length for single mode fibers and erbium doped fiber, respectively. In this equation, ���corresponding to standard step-index single mode fiber provides extensive anomalous dispersion (negative), whereas that of erbium doped fiber shows normal dispersion (positive). Since single mode fibers are always dominating in the construction, the total cavity dispersion shows to be negative.

For saturable absorber, here we use Carbon Nanotube (CNT) thin film instead of Semiconductor

Saturable Absorber (SSA). SSA has a relatively long recovery time and is typically designed for reflection in Fabry-Perot cavity. However, CNT has a faster recovery time smaller than one picosecond and is suitable for transmission. Therefore CNT is more desirable in this case.

In this experiment, CNT is provided by Imperial University, which is pre-adjusted to have the maximum absorption around 1550nm to meet the desired operating wavelength of the laser. A simple saturable absorber model is [2]:

(2)

where I is the intensity of pump power, �ns is non-saturable absorption coefficient, Isat is defined as saturation intensity, which is the optical intensity in the steady state leads to a reduction in the gain to half (or -6dB) of its small-signal value. It turns out that the mathematical model of CNT is very similar to that of gain media.

Generally, soliton systems have the highest stability margins [3]. Characteristics of soliton lasers are sech2-shaped pulses with long extended temporal pulse wings and spectral sidebands (or called Kelly sidebands) due to the interaction of the soliton pulses with a weak background that accompanies soliton pulse generation [3]. Soliton pulses arise from a cancellation of phase perturbation due to dispersion and self-phase modulation (SPM) in a negative-dispersion fiber: SPM leads to up-chirped pulse, while anomalous dispersion leads to down-chirped pulse.

According to the equation:

f=c/(n*LTotal) (3)

To meet the objective of increasing pulse repetition frequency, total fiber length in the setup need to be shortened as ,much as possible in the classical model

C. EXERIMENTAL SETUP

Fig. 2. Experimental Setup

Figure 2 showed above is the experimental setup. Being different from the simulation model, this is a simplified one to make the laser system more compact

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tects the SWNT, which is pla

duced. Hence pulse repetition frequency is increased.

D.

shows the pulse repetition frequency to be 18.6MHz.

and hence greatly increase the pulse repetition frequency. In this setup, we adopt a newly developed component called tap isolator wavelength division multiplexer (TIWDM). TIWDM is a three-in-one device integrating the functionality of the isolator, pump/signal wavelength division coupler (WDM) and a 10% output tap coupler. Besides TIWDM, an additional wavelength division multiplexer (WDM) is adopted to implement backward pump. It gives higher pump efficiency and also pro

ced inside the connector. By replacing the three components with one

TIWDM, fusion splices are greatly reduced from six to two. Thus better loss control id achieved. And due to the constriction introduced by fusion splice machines, every time of fusion splice requires about 20cm of fiber length. Therefore, total fiber length is also greatly

Fig. 4. Pulse repetition frequency of 180MHz

To further increase the repetition frequency in future study and experiments, further work can be done:

1). Use new type of fiber gain material with higher gain to reduce the fiber length.

2). Directly coat CNT to one end of EDF, thus reduce one fiber connector. re

3). Change to a better laser source so that the additional WDM can be removed. EXERIMENTAL PROCEDURES AND RESULTS

After the proper construction of the setup, we connect the input to a 980nm laser source with adjustable input power and measure the output pulse repetition frequency. With fine adjustment of the polarization controller, a maximum bandwidth could be obtained while still maintaining the soliton laser state. Figure 3

Following are Some conclusions:

a). Unstable output occurred when pump power was low.

b). At a certain input power level(about 60 mW in this setup), stable mode-locking self-started. The laser could mode-lock at several different wavelengths. These could be selected by changing the settings of the polarization controller. Around 1554 nm pulse was the most stable against polarization perturbations.

c). When pump power increased to a certain threshold, the laser started operating in a multi-pulse mode. However, stable mode-locking could be achieved again as soon as the pump power was brought back to below this threshold.

ACKNOWLEDGMENT Fig. 3. Pulse repetition frequency of 18.6MHz

In order to further increase the pulse repetition frequency, we cut off some part of the single mode fiber and fusion splice again. Then a pulse repetition frequency of 50.9MHz is obtained. Repeating the same procedure,

I wish to acknowledge the funding support for this project from Nanyang Technological University under the Undergraduate Research Experience on Campus (URECA) programme.

REFERENCESa pulse repetition frequency of 78.6MHz is

obtained.

gure 4. At this stage, total fiber length was 110cm.

[1] G. P. Agrawal, Nonlinear fiber optics 3rd edition, 2001.

[2] F. Wang, A. G. Rozhin, Z. Sun, V. Scardaci, I. H. White, and A. C. Ferrari, "Soliton fiber laser mode-locked by a single-wall carbon nanotube-polymer composite", PSS, 29 August 2008.

IV. CONCLUSION

Finally, we successfully demonstrated a high repetition frequency passively mode-locked erbium doped fiber ring laser using SWNT and a multi-functional device TIWDM. With TIWDM, repetition frequency could be increased to a new stage. Pulses with a fundamental repetition frequency of 180MHz and temporal duration of 500 femtosecond (5×10-13)were generated, as shown in Fi

[3] Martin E. Fermann and Ingmar Hartl, "Ultrafast Fiber Laser Technology", IEEE Journal of Selected Topics in Quantum Electronics, VOL. 15, NO. 1, January/February 2009.