540 fs Light Pulses at 1.5 pm with Variable ... single mode fiber is used to generate light pulses of 3 ps width. ... or to 540 fs pulses with a small pedestal.
IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 6, NO. 10, OCTOBER 1994
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540 fs Light Pulses at 1.5 pm with Variable Repetition Rate using a Tuneable Twin Guide Laser and Soliton Compression in a Dispersion Decreasing Fiber M. Schell, D. Bimberg, V. A. Bogatyrjov, E. M. Dianov, A. S. Kurkov, V. A. Semenov, and A. A. Sysoliatin
Abstract-A novel and simple method to generate ultra short light pulses with variable repetition rate is demonstrated. A wavelength modulated tuneable twin guide laser, emitting bluechirped light, together with pulse compression in a standard single mode fiber is used to generate light pulses of 3 ps width. Amplification and further compression in a dispersion decreasing fiber leads to Fourier transform limited pulses of 1.87 ps width or to 540 fs pulses with a small pedestal.
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Manuscript received May 4, 1994; revised July 18, 1994. This work was partially supported by the DFG (Bi 284/8-1). M. Schell and D. Bimberg are with the Institut fur Festkarperphysik I, Technische Universitat, Berlin, Germany. V. A. Bogatyrjov, E. M. Dianov, A. S. Kurkov, V. A. Semenov, and A. A. Sysoliatin are with the General Physics Institute, Russian Academy of Sciences, Moscow, Russia. IEEE Log Number 9404992.
by using a single laser without the need for any sopisticated temperature control, and most importantly with the possibility to electrically trigger and tune the repetition frequency of the optical pulses. The experimental set up and the voltage pulses used to drive the laser diode are illustrated in Fig. 1 . The TTG laser is coupled into a SMF either directly or via a 600 l/mm grating. An approximately rectangular electrical pulse of 2 ns width is applied to the lasing region. About 1 ns later, after the relaxation oscillations have disappeared, a pulse of 1 ns width is applied to the tuning layer of the TTG laser, shifting the emission linearly to shorter wavelengths. The purpose of the grating is to eliminate both the emission during the switch-on process and the rise and the fall of the electrical tuning pulse, which is not linearly chirped. The output is compressed in 17.2 km SMF (Siecor E9/125) with approximately -300 ps/nm dispersion. The resulting pulse is amplified in an erbium doped fiber amplifier (EDFA, BT&D EFA 4000) and coupled into 910 m of the DDF. The DDF is drawn from a standard preform, varying the outer diameter from 190 pm to 143 pm with a deviation less than 0.3%, due to the in-situ control of the fiber diameter [lo]. The dispersion (1 - 0 . 9 d m ) decreases according to D ( z ) = lo& for 0 < z < 840 m and is constant at I& for 840 < z < 910 m. The repetition rate is deliberately kept as low as 1 MHz due to the low saturation energy of our EDFA. A repetition rate of 600 MHz without soliton compression was already demonstrated [ l l ] , and an increase to several GHz should be possible using a TTG-laser optimized for fast wavelength
RESENTLY, only rather complex mode-locking [l], [2] (passive or hybrid) techniques are available for sub-ps pulse generation using diode lasers. The pulses are generated at a fixed repetition rate, given by the resonator roundtrip time. For many applications in data transmission or measurement systems, however, direct data modulation and the tunability of the repetition rate are desirable. Another interesting method, gain switching [3] of a DFB laser diode, followed by compression of the red chirped output allows the generation of pulses of a width downto 2 ps [4], but suffers from two other disadvantages: (i) Gain switching of single mode laser diodes is connected with a large timing jitter [ 5 ] . The time jitter can be decreased by biasing the laser near threshold, which decreases the chirp and increases the pulse width, and hence also increases the pulse width after compression, however. (ii) The amount of linear chirp, which is necessary for compression, can not arbitrarily be increased, so that sub-ps pulse generation using this technique has not yet been reported. In this letter, we report for the first time the generation of light pulses at 1.55 pm by compression of a linearly bluechirped tuneable twin guide (TTG) laser [6] in a standard single mode fiber (SMF) and in a decreasing dispersion fiber (DDF), using the dependence of the first order soliton energy on the fiber dispersion [7], [8], [9]. The theoretical principles of soliton compression are described by Mamyshev er al. IS]. The main advantage of our method is its extreme simplicity
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 6, NO. IO, OCTOBER 1994
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tuning. The final pulse is analysed using an autocorrelator in noncollinear configuration with subsequent second harmonic generation (SHG) in LiNbO3, and a monochromator with 0.26 nm resolution. The autocorrelation signals measured after the SMF are shown in Fig. 2. The continuous line is obtained without the grating. Due to the red-chirped emission during the switch on of the laser a broad pedestal of about 200 ps width is present. Additionally, the nonlinear chirp occumng during the rise and decay of the tuning pulse causes a small secondary pulse at a distance of about 5 ps from the main pulse, visualized by the shoulders of the main pulse in Fig. 2. Neglecting these shoulders the SHG-trace is fit very well by a sech2pulse shape of 1.9 ps FWHM with a pedestal. The unwanted pedestal and secondary pulses are removed by spectral filtering using the grating. A parallel beam is incident to the grating, resulting in a spectral width of the back reflection of 1.0 nm. Due to the time delay between the opposite edge rays of the light beam reflected from the grating the pulse is broadened, however. A perfect fit is obtained by a sech2-pulse profile with 3.5 ps FWHM without any background. A peak power of 60 mW is obtained. In Fig. 3 the autocorrelation curves after the EDFA and the DDF are shown. In addition, a third curve is shown, where the collimating objective was adjusted to create a convergent beam. As a consequence the size of the illuminated spot on the grating reduced to half as compared to a parallel beam, resulting in less temporal broadening of the pulse. The suppression of the background is deteriorated by a convergent beam, however. The results can be interpreted as follows. Without the grating the central peak of Fig. 2 is shortened due to the nonlinear effects in the fiber. The second pulse in Fig. 3 corresponds to the shoulder in Fig. 2 and was not shortened, because its energy is significantly lower than the first order soliton energy at the beginning of the DDF. If the second pulse is neglected, the main pulse can perfectly be fit by a sech2-pulse with 540 fs width, corresponding to a shortening by about a factor of 3.5. If the grating is illuminated with a convergent beam the pulse broadens to 850 fs due to the broadening of the initial pulse, and both the pedestal and the second pulse decrease drastically. This observation proves that the second pulse was part of the initial pulse and is not caused by nonlinear effects in the DDF, which occur e.g. in compression experiments using third order solitons [ 121, [ 131 rather than fundamental solitons as in this work. If
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the background and the second pulse are removed completely using a parallel beam incident to the grating the pulse broadens to 1.87 ps, assuming again a sech2-shape. Hence for these input pulses only a compression by a factor of 1.9 is obtained. We think that the comparatively small compression is due to the fact that the input pulse now is too broad to allow for adiabatic compression. Note that with increasing pulse width the fundamental soliton energy decreases, resulting in a reduction of the nonlinear effects necessary for adiabatic compression. This effect can be accomodated using a longer DDF. Simulations show that a DDF with the same dispersion parameters, but a length of about 3 km should be sufficient to also compress the broader, background-free input pulses. Such experiments will be carried out soon. To find out whether the pulses are solitons, both the temporal shape and the spectrum have to be analysed. If convergent focusing onto the grating or no grating is used the spectra deviate from the spectrum of an ideal soliton due to the presence of the background components. Hencle the soliton character can not be proven. On the other hand the temporal pulse shape can be fit very well with a sech2-shape in both cases. The spectra for a parallel beam incident to the grating are shown in Fig. 4 for two different values of the EDFA amplification. The background visible in Fig. 41 stems from the EDFA. If the amplification is low (dashed line) the spectral width and pulse width are 1.15 nm and 2.96 ps, respectively. This corresponds to a time bandwidth product of 0.42. which is above the Fourier transform limit of 0.31. If the pulse energy is increased by a factor of 5 the pulses become shorter (1.87 ps, Fig. 3), with approximately the same spectral width (0.97 nm).
SCHELL er oL: 540 fs LIGHT PULSES AT 1.5 pm WITH VARIABLE RECEFTION RATE
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The resulting time bandwidth product is 0.23, which is lower Amplification and further compression in a decreasing disthan the Fourier-limit for sech2-pulses. The pulse energy is in persion fiber leads to 540 fs pulses with pedestal and to the range 3-10 pJ, which roughly coincides with the calculated Fourier-limited 1.87 ps-pulses without pedestal. The main pulse energy for a first order soliton of 3 ps width (4 pJ). advantages as compared to other methods are (i), the tunability The large uncertainity in the pulse energy is caused by the of the optical pulse repetition frequency, (ii), the low time jitter subtraction of the background generated by the EDFA. The and (iii), the use of a commercial laser diode without the need odoff ratio of our pulses, determined from the autocorrelation for temperature control. traces, is better than 100, while the signal-to-noise-ratio ( S N R ) defined as pulse energy divided by noise energy is 5% only, ACKNOWLEDGMENT due to the low duty factor (1:500.000) caused by the low The authors are indebted to A. Gladisch and H. Burkhard repetition frequency of 1 MHz. The positioning of the spectral (German Telekom) for supplying the EDFA, to S. Y. Wang filter after the EDFA, which could not be done in our set-up (HP Palo Alto) for supplying a fast photodetector, to D. Huhse for technical reasons, would suppress the largest part of the and W. Utz for their support, and to Dr. N. N. Bubnov for background generated by the EDFA, and an increase of the stimulating discussions and his support. repetition frequency to 100 MHz would additionally increase the SNR by a factor of 100. REFERENCES We anticipate that our pulses show an extremely low time [ l ] A. G. Weber, M. Schell, G. Fischbeck, and D. Bimberg, “Generation of jitter. In an earlier investigation of compressed TTG-laser single femtosecond pulses by hybrid mode locking of a semiconductor pulses a rms time jitter smaller than 100 fs (3 kHz < laser,” IEEE J. Quantum Electron., vol. 28, pp. 2220-2229, 1992. A f < 1 MHz) was determined [I 11. The low jitter compared [2] D. J. Derickson, et al., “Short pulse generation using multisegment mode-locked semiconductor lasers,” IEEE J. Quantum Electron., vol. to pulses from gain switched single mode laser diodes re28, pp. 2186-2202, 1992. sults from the fact, that the emission time is determined by [3] D. Bimberg, K. Ketterer, E. H. Bottcher, and E. Scholl, “Gain modulathe wavelength modulation, when the laser is already above tion of unbiased semiconductor lasers: ultrashort light-pulse generation in the 0.8 pm-1.3 p m wavelength range,” Inr. J. Electronics, vol. 60, threshold. Therefore fluctuations of spontaneous emission, pp. 23-45, 1986. which cause large instantaneous timing jitter [5] in gain Liu, Y. Ogawa, and S. Oshiba, “Generation of an extremly short [4] H.-F. single mode pulse (-2 ps) by fiber compression of a gain-switched pulse switching experiments, are not significant in our method. from a 1.3 p m distributed-feedback laser diode,” Appl. Phys. Lett., vol. In our present set-up, a total loss of 17 dB from the 59, pp. 12861286, 1991. laser facet to the end of the DDF was measured for a cw, [5] A. G. Weber, W. Ronghan, E. H. Bottcher, M. Schell. and D. Bimberg, “Measurement and simulation of the turn-on delay time jitter in gainmonochromatic signal, if no EDFA was used. This loss is due Quantum Electron., vol. 38, switched semiconductor lasers,” IEEE J. to the laser-fiber coupling (8.2 dB loss including the grating), pp. 441-445, 1992. the non-optimised coupling to the DDF (3 dB), the loss in 161 M.-C. Amann. S. Illek, C. Schanen and W. Thulke, “Tuning range and threshold current of the tunable twin-guide (TTG) laser,” IEEE Photon. the SMF and the DDF (3.2/0.5 dB) and a number of fiber Technol. Lett., vol. 1, pp. 253-254, 1989. connectors (2.1 dB). Additionally, a TTG laser with low output [7] K. Tajima, “Compensation of soliton broadening in nonlinear optical power, 6 mW at 120 mA cw drive current is used. If the output fibers with loss,” Optics Lett., vol. 12, pp. 54-56, 1987. [8] P. V. Mamyshev, S . V. Chemikov and E. M. Dianot, “Generation of power of the TTG laser could be increased to 40 mW during fundamental soliton trains for high-bit-rate optical fiber communication the 2 ns driving pulse, and assuming that the blue chirp is lines,” IEEE J. Quantum Electron., vol. 27, pp. 2347-2355, 1991. linear only during 0.5 ns, pulses with an energy of 20 pJ [9] S. V. Chemikov, E. M. Dianov, D. J. Richardson, R. I. Laming and D. N. Payne, “114 Gbit/s soliton train generation through Raman selfcould be generated. If additionally the loss could be decreased scattering of a dual frequency beat signal in dispersion decreasing fiber,” to 7 dB (3 dB SMF, 2 dB laser fiber coupling, 2 dB fiber A&. Phys. Lett., vol. 63, pp. 293-295, 1993. Fabry-Perot for background suppression) the soliton energy of [lo] V. A. Bogatyrjov, et al., “A single-mode fiber with chromatic dispersion varying along the length,” IEEE J. of Lighrwave Tech., vol. 9, pp. 4 pJ for 3 ps pulses could be reached even without the use 561-566, 1991. of an EDFA, which would be a great advantage concerning [ I l l M. Schell, D. Huhse, and D. Bimberg, “Picosecond pulse generation with a 1.55 p m tunable twin guide laser using blue-chirp compression,” complexity and expenses of the set-up.
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CONCLUSION In conclusion, we have demonstrated a novel and simple method to generate ultrashort light pulses using blue chirp generation and compression in a standard single mode fiber.
Appl. Phys. Lett., vol. 64,no. 15, pp. 1923-1925, Apr. 1994. [12] H.-F. Liu, Y. Ogawa, S. Oshiba, and T. Nonaka, “Picosecond pulse generation from a 1.3 p m DFB laser diode using soliton-effect compression,” IEEE J. Quantum Electronics, vol. 27, pp. 1655-1659, 1991. [13] J. T. Ong, R. Takahashi, M. Tsuchiya, Y. Ogawa, and T. Kamiya, “Soliton compression (-1.2 ps) of gain-switched DFB laser using an erbium-doped fibre amplifier,” Proc. 13th IEEE Int. Semicond. Laser ConK, Takamatsu, paper D33, 1992.
