OPTICAL FILTERING OF HIGH-SPEED MODULATED SEMICONDUCTOR LASER

Przemysław Krehlik, Łukasz Śliwczyński

Abstract – In the paper we demonstrate that optical filtering may significantly increase the dispersion tolerance of high-speed optical signal, emitted by the directly modulated laser. As it was found, quite good results may be obtained by locating the laser optical spectrum just at the pass-band edge of the highly selective filter. Both simulations and measurement results, illustrating the usefulness of proposed scheme, are shown.

Index Terms –directly modulated laser, optical filtering, dispersion tolerance.

I. INTRODUCTION

A directly modulated laser (DML) exhibits so-called chirp, i.e. the instantaneous optical frequency variations associated with the intensity modulation. Usually we distinguish two different chirp components: the adiabatic one, which describes the frequency deviation proportional to the actual laser output power, and transient one, connected with the time-derivate of output power [1]. The chirp causes serious broadening of emitted signal spectrum, which interplays with the fiber chromatic dispersion causing time-domain distortions of transmitted signals. In case of 10 Gb/s data transmission in the 1.55 µm window the standard fiber dispersion together with the laser chirp usually limit the transmission distance to not more than a dozen kilometers or so.

Since the chirp broadens the spectrum of the modulated signal, the optical filtering seems to be promising method for reducing its impact on transmission limitations. Some attempts in this area, as narrowband, and/or detuned, and/or cascaded filtering of chirped signal were reported in previous works [2-7]. However, to our knowledge, the substantial advantage in term of dispersion tolerance, observed when the laser spectrum is located on the proper edge of the filter pass band, was not clearly pointed out before.

Because the transmission limitations steaming from chirp and dispersion grow rapidly with increasing modulation speed, the case of 10 Gb/s modulation, being the highest practically used in direct modulation, will be taken in our investigations.

II. OPTICAL FILTERING OF LASER OUTPUT SIGNAL

To illustrate the impact of the optical filtering on laser output signal we take a narrow band-pass filter as an example (Fig. 1). It should be stressed that not only the

magnitude, but also the phase (or equivalently the group-delay) characteristics affect the transmitted signal. The results of filtering, for laser spectrum located at different points on the filter characteristic, are presented in form of eye diagrams in Fig. 2. The 10 Gb/s eye diagrams observed at both filer output and after 50 km long fiber, displaying 17 ps/(nm·km) dispersion, are presented.

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Figure 1. Typical characteristics of band-pass optical filter. Three different locations of laser spectrum, corresponding to eye patterns shown in Fig. 2, marked by the arrows.

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Figure 2. Eye patterns observed at filter output and at 50 km-long fiber output for different laser spectrum locations. (Simulation.)

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In a typical case, when the laser spectrum is located at the center of the filter pass band (see the gray arrow in Fig. 1 and corresponding eye patterns in Fig. 2 a), the laser output signal is affected the least by the filtering, because of the flat magnitude response and nearly constant group delay in this region.

Thus, the eye pattern at the filter output is quite clear, but at the fiber end it is strongly corrupted by the fiber dispersion. Shifting the laser spectrum a bit towards higher frequencies (Fig. 2 b) we can observed the situation that may be regarded as dispersion precompensation; the filter magnitude response is still flat, and its rising group delay acts similarly to a dispersion (pre)compensator. Thus the eye pattern at the fiber output may be quite clear.

This possibility of using pass-band filter as dispersion (pre)compensator was reported previously [2]. However, the method has serious disadvantages. First, the group delay rising is inherently nonlinear, which makes perfect dispersion compensation impossible. .Second, the compensation is matched only to certain dispersion (fiber distance), and so is improper for the other values.

Other interesting case, to our knowledge not widely described before, is to locate the laser spectrum at the left edge of the filter pass band (Fig. 2 c). This time both the magnitude and group-delay characteristics of the filter play important role in the signal processing. What seems to be strange, now the eye pattern is quite clear at both the filter output and at the end of the dispersive fiber as well. For some explanation it is worth to analyze not only the optical power waveforms, but also the time evolution of optical instantaneous frequency variations (chirp) – Fig. 3. As may be noticed the power waveform is only slightly affected by the filtering, but the chirp is modified substantially. It may be noticed that the adiabatic part of the chirp is left almost intact whereas the dynamic chirp at the rising edges is greatly reduced. The chirp associated with the falling edges may be perceived as being delayed on the other hand. All these together cause that during the slopes and the periods when the power is high, the signal is affected by almost constant chirp (i.e. has constant frequency). These parts of the signal are traveling with almost constant speed and thus not suffer much from the dispersion. During remaining periods of time the signal is in the low state and conveys much less of its energy, thus the chirp is not of great importance there.

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Figure 3. Intensity and chirp waveforms before (a), and after (b) filtering.

Extended simulation investigations showed that similar effect of enhanced dispersion tolerance may be obtained

also in band-stop or high-pass filters, when the laser spectrum is located at analogous edge of the transmission band – see Fig. 4. The best results were observed for rather sharp filters with slope in range of 4 ... 6 dB/GHz (measured at –3 dB attenuation), and group delay peaking of approximately 50 ps. The lasers with generally low chirp are preferred, but there are not any particular demands in this matter. Additionally, the optimum location of laser spectrum is only weekly dependent on the fiber dispersion, and thus its constant location may be used in practice for wide range of the dispersion.

Figure 4. Desired location of the laser spectrum related to the filter transmission band.

III. EXPERIMENTAL RESULTS

To verify the results of our simulations the hardware experiment was performed. The 1.55 µm laser (NLK 1551) was modulated by the 10 Gb/s pseudorandom bit generator. Differently to the example filter shown in Fig. 1, this time we take the band-stop filter – namely the Bragg filter configured in the transmissive mode (Fig. 5). Some spools of standard single-mode fibers were used as a transmission medium. The optical amplifier and high-speed sampling oscilloscope were used to register the signal at the fiber output. The laser was thermally tuned close to the filter edge, to obtain the best eye opening for 100 km long fiber, and its wavelength was not modified for other distances.

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Figure 5. Characteristic of the used Bragg filter, with laser spectrum location marked with the arrow.

Obtained results are shown in Fig. 6. Quite good eye opening was observed for all fiber distances in case of filtered signal whereas without the filter the eye was destroyed even in case of relatively short, 30 km fiber. Additionally, some improvement of the extinction ratio, caused by the FM to IM conversion of the laser chirp, occurring at the filter slope, may be observed.

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Figure 6. Eye patterns registered with different fiber length, without filter (left column) and with filter (right column).

IV. LASER WAVELENTH LOCKING

Some problem in practical application of proposed filtering is that good dispersion tolerance may be achieved only for quite precise location of laser optical frequency at the filter pass band edge. Generally, accuracy better then ±1 GHz is required. Additionally, because any filter has some temperature susceptibility, the laser should be locked to actual filter characteristic, not to some other frequency etalon. Thus, the best solution is to use three-port optical filter, as Bragg filter combined with a circulator, or thin-film filter with both transmissive and reflective outputs. This way the portion of the laser power attenuated at the main output port will be present at the auxiliary port and forms the signal for closed-loop wavelength stabilization, as shown in Fig. 7.

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Figure 7. Locking of the laser wavelength at the filter pass-band edge.

V. CONCLUSION

In the paper we present the method of increasing dispersion tolerance of high-speed modulated optical signal, emitted by the directly modulated laser. The method is based on reshaping the chirp characteristics of the signal by its optical filtering at the filter pass-band edge. The main advantage of the proposed idea, comparing to standard dispersion compensation methods, is that correct fiber output signal may be obtained in wide range of

accumulated fiber dispersion, without any trimming of the transmitter. Additionally, the proposed filtering does not introduce any noticeable attenuation of the signal. Some practical issues, which should be taken into account, are that quite sharp filter is needed, and that the laser wavelength locking should be arranged. Presented measurement results illustrate the usefulness of proposed filtering method.

REFERENCES

[1] K. Peterman: Laser diode modulation and noise, Klüwer, Dordrecht 1991,

[2] B. J. Eggleton, T. Stephens, P. A. Krug, G. Dhosi, Z. Brodzeli and F. Ouellette, “Dispersion compensation using fibre grating in transmission”, Electronics Letters , vol. 32, pp. 1610-1, 1996.

[3] P. A. Morton, G. E Shtengel, L. D. Tzeng, R. D. Yadvish, T. Tanbun-Ek, R. A. Logan, “38.5 km error free trans-mission at 10 Gbit/s in standard fibre using a low chirp, spectrally filtered, directly modulated 1.55 mm DFB laser”, Electronics Letters, vol. 33, pp 310-1, 1997,

[4] S. B. Park, C. H. Lee, “Enhancement of system performance in directly modulated metro-WDM systems by a spectral filtering method”, Electronics Letters, vol. 38, pp. 418-9 2002,

[5] Y. Matsui, D. Mahgerefteh, X. Zheng, C. Liao, Z. F. Fan, K. McCallion at all., “Chirp-managed directly modu-lated laser (CML)”. IEEE Photonics Technology Letters vol. 18, pp. 385 – 7, 2006,

[6] I. Tomkos, R. Hesse, N. Antoniades and A. Boskovic, “Filter Concatenation in Metropolitan Optical Networks Utilizing Directly Modulated Lasers”, IEEE Photonics Technology Letters, vol. 13, 2001

[7] L. S. Yan, Y. Wang, B. Zhang, C. Yu, J. McGeehan, L. Paraschis at all., “Reach extension in 10 Gb/s directly modulated transmission systems using asymetric and narrowband optical filtering”, Optics Express, vol. 13, pp. 5106-15, 2005.

AUTHORS’ NOTE

Przemysław Krehlik – AGH University of Science and Technology, Mickiewicza 30 Ave., 30-059 Kraków, e-mail: krehlik@agh.edu.pl

Łukasz Śliwczyński – AGH University of Science and Technology, Mickiewicza 30 Ave., 30-059 Kraków, e-mail: sliwczyn@agh.edu.pl

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