Femtosecond pulse generation by actively modelocked fibre ring laser IV
2 ns
D.T. Nguyen, A. Muramatsu and A. Morimoto
a
Femtosecond optical pulses are generated by an actively modelocked fibre ring laser. The fibre ring cavity includes a dual-electrode MachZehnder optical intensity modulator and an erbium-doped fibre amplifier. To generate femtosecond pulses, fast impulse modulation is employed. The modelocking generates femtosecond pulses with a pulse-width of about 500 fs.
Introduction: In recent decades, with the development of various erbium-doped fibres, fibre lasers have received much attention, because of the numerous advantages such as simple doping procedures and low loss [1]. Some different cavity configurations can be built with fibres and fused-fibre couplers, including linear Fabry-Pérot, ring, and combinations of both. Among those configurations, fibre ring lasers have been widely investigated [2–5]. In addition to offering continuous-wave operation, fibre lasers can be modelocked to generate ultrafast optical pulses. Modelocked fibre lasers are valuable sources of high-quality ultrafast optical pulses. Passively modelocked fibre lasers can generate femtosecond pulses [3]. However, the passively modelocked fibre lasers are sensitive. Once the stable passive modelocking is achieved, a relatively narrow parameter range is acceptable. Actively modelocked fibre lasers are capable of producing transform-limited optical pulses with variable pulse-widths in the picosecond range and multi-GHz repetition rates [5, 6]. Recently, the employment of an all-fibre acousto-optic modulator [7] or an electro-optic modulator [8] in an actively modelocked fibre ring laser has been reported. These setups generate relatively long pulses in subnanosecond range. In this Letter, we describe a simple method to generate femtosecond optical pulses by an actively modelocked fibre ring laser based on the employment of impulse modulation.
V1
V2
eff. driving voltage (V1–V2)
b
t
Fig. 2 Measured result of two driving signals fed into two electrodes of intensity modulator (Fig. 2a), and modelled effective driving signal (Fig. 2b)
Impulse modulation: To explain the operation of the employed impulse modulation, the impacts of two driving signals on two electrodes can be modelled by an effective driving signal as shown in Fig. 2b. With 1 ns delay-time, the effective driving signal has short peaks (in subnanosecond range). Fig. 3 shows the measured transmission factor of the intensity modulator against applied DC bias. The effective driving signal modulates the transmitted intensity about the bias point. The inset of Fig. 3 shows the output temporal transmission profile after the impulse modulation with two ultrafast optical pulses. These pulses will be amplified by the EDFA then go around the fibre ring cavity. After the modelocking, only the greater pulse (in two generated pulses) survives in the ring cavity. In this case, two generated pulses are the same amplitude and by tuning the optical bias, after the modelocking, both pulses exist in the laser cavity. These analyses are experimentally demonstrated in the following Section.
Experimental setup: The experimental setup of the actively modelocked fibre ring laser is shown in Fig. 1. The fibre ring laser consists of a dual-electrode Mach-Zehnder intensity modulator (T-DEH1.5-20-ADC), an EDFA (AMP-FL8013-CB-13), a polarisation controller (PC), and a 10/90 output coupler. The output is characterised by an autocorrelator, a spectrum analyser, and an oscilloscope.
transmission factor 1
transmitted intensity
Vπ
t –8 –7 –6 –5 –4 –3 –2 –1 0 1 2 3 4 5 6 7 8
10
eff. driving voltage
output
90
PC
EDFA
coupler 10/90
t
Fig. 3 Measured transmission factor of intensity modulator against applied DC bias; the applied effective driving voltage modulates the transmitted intensity about the bias point.
V1 AM modulator DC bias DC supply
DC bias (V)
V2 variable delay
function generator amplifier
Inset: Output temporal transmission profile after impulse modulation 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 –0.1 –10 –8
In actively modelocked fibre lasers, to shorten optical pulses, fast and large modulation is required. Here, we employed impulse modulation to achieve femtosecond optical pulse generation. The impulse modulation operates based on driving two electrodes of the intensity modulator by two electrical pulse trains generated by an electrical pulse generator. The electrical pulse generator consists of a function generator (33250A) and a power amplifier (AA180-RS) working in saturation mode. By using a broadband power splitter, the output of the amplifier is divided into two arms with one delayed by a variable delay (Fig. 1). In our experiment, we used the shortest electrical pulses generated by the function generator, which have a pulse-width of 8 ns and a risetime of 5 ns. Fig. 2a shows the measured result of two driving signals fed into two electrodes of the intensity modulator, where the delay-time was 1 ns. After the amplifier, electrical pulses from the function generator are amplified and shaped to sharper driving pulses with a risetime of about 2 ns (calculated from 10 to 90 % of the pulse amplitude).
intensity, a.u.
Fig. 1 Experimental setup of actively modelocked fibre ring laser
–6 –4
–2
0 2 time, ps
4
6
8
10
Fig. 4 Experimental autocorrelation trace; pulse-width is about 500 fs if assumed pulse profile is sech2
Results and discussion: The round-trip frequency of the employed fibre ring cavity is fm = 9.188 MHz. In our experiment, the impulse modulation was carried out at this fundamental frequency. The variable delay was set at 1 ns. The DC bias of the intensity modulator was set at 2.5 V. The output power of the EDFA was about 20 mW. The output ultrafast optical pulses were measured by an autocorrelator (HAC-150). Fig. 4 shows the experimental autocorrelation trace. The
ELECTRONICS LETTERS 11th April 2013 Vol. 49 No. 8
autocorrelation trace width is about 770 fs (FWHM). By assuming sech2 pulse profile, the pulse-width is about 500 fs. The output spectrum is displayed in Fig. 5 with a 3 dB spectral bandwidth of about 0.38 THz.
pulse-width, fs
1000
–30
800 600 400 200
intensity, a.u.
0
–40
0
0.5
1.0
1.5
2.0 2.5 delay, ns
3.0
3.5
4.0
4.5
Fig. 7 Relationship between delay-time and pulse-width of output optical pulses
–50 194.9
195.9 frequency, THz
196.9
Fig. 5 Output spectrum; FWHM spectral bandwidth is about 0.38 THz
A fast photodiode connected to an oscilloscope was used to characterise the output optical pulse trains. Fig. 6 shows the output optical pulse trains at different bias points of the impulse modulation. As analysed in the preceding Section, after impulse modulation two optical pulses with different amplitudes were generated when DC bias is set at 2.5 and 2.9 V. Therefore, after modelocking only one optical pulse exists in each round-trip of the laser cavity as shown in Figs. 6a and c, respectively. Meanwhile, two optical pulses with the same amplitude were generated when the DC bias was set at the minimum transmission point of 2.7 V. As a result, after modelocking both optical pulses still survive as shown in Fig. 6b.
5 ns
a
b
c
Fig. 6 Output optical pulse trains with DC bias set at 2.5, 2.7, and 2.9 V, respectively a 2.5 V b 2.7 V c 2.9 V
Furthermore, the influence of the delay-time of the variable delay on the pulse-width of the output optical pulses is studied. This relationship is shown in Fig. 7. As the delay-time increases from 1 to 4 ns, the pulsewidth increases from 500 to 800 fs. This increase can be attributed to the fact that the peaks of the effective driving signal will widen as the delaytime increases. As a result, the modelocking will generate longer pulse duration.
Conclusion: We have proposed a simple method to generate femtosecond optical pulses by an actively modelocked fibre ring laser. The operation of this laser is realised by an optical intensity modulator and an EDFA. Impulse modulation is employed to generate femtosecond pulses. The experimental results show that the optical pulses with duration of about 500 fs are obtained. © The Institution of Engineering and Technology 2013 25 December 2012 doi: 10.1049/el.2012.4483 One or more of the Figures in this Letter are available in colour online. D.T. Nguyen, A. Muramatsu and A. Morimoto (Department of Photonics, Ritsumeikan University, 1-1-1 Nojihigashi, Kusatsu, Shiga 525-8577, Japan) E-mail:
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