MULTIFREQUENCY CONTINUOUS WAVE ERBIUM DOPED FIBER NON-RESONANT OPTICAL SOURCE

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Piotr STĘPCZAK*

MULTIFREQUENCY CONTINUOUS WAVE ERBIUM DOPED

FIBER NON-RESONANT OPTICAL SOURCE

The simulation and experimental results of non-resonant comb source properties are presented. In the experiment, eleven 1,5 GHz spacing wavelengths were obtained, however, the simulations resulted in 31 peaks separated by 3 GHz generation.

Keywords: Multi-wavelength source, wavelength division multiplexing, erbium doped fiber

1. INTRODUCTION

Optical multifrequency sources play an important role in metrology, scientific and industrial applications. Broad homogenous gain of the erbium doped fiber (EDF) is the main problem to overcome in EDF based laser multiwavelength configurations [1-4].

In this paper, we focus on a non-resonant, ring optical frequency comb generator [5] (Fig.1) employing a three port acousto-optic frequency shifter (AOFS), a single master laser (ML), an erbium doped fiber amplifier (EDFA) and a band limiting filter (BLF). Every loop round-trip AOFS splits the optical beam into two and the frequency shifted ray after the amplification and filtration is directed back to the AOFS input. The multiple divisions, spectrum shifting and the optical beam amplification in the ring configuration result in the generation of an optical frequency comb. The AOFS controlled by a RF generator determines the frequency comb interval and the BLF limits the number of frequency lines.

Fig. 1. Configuration of a single shifter, multiwavelength erbium doped fiber ring source

2. SIMULATION RESULTS

The simulation process was performed using rate equations, Giles model of EDFA and mathematical models of other optical components (AOSF, ML, BLF) with regard to the actual parameters of the commercially available modules. The simulations allowed a full spectral analysis of a source operation. The results of calculations of an optimised single AOFS configuration (Fig. 1) with the interchannel space of 1,5625 GHz and 31 spectral lines are shown in Fig. 2-4.

Fig. 2. Power spectrum consisting of 31 lines, channel separation: 1,5625 GHz The pick power variation over 31 channels was in the range of 0,15 dB. Fig. 3 shows noise properties of the generated optical comb.

Fig. 3. OCNR of 31 lines generation, channel separation: 1,5625 GHz

In order to compare the experimental and theoretical results and their compatibility, we calculated an output spectrum for eleven optical carriers (Fig. 4).

Fig. 4. Power spectrum consisting of 11 lines

We also analysed the operation of a double shifter configuration which allowed to obtain a 3,125 channel distance. To compensate the losses, it was necessary to implement two EDFA modules (Fig. 5).

Fig. 5. Configuration of a double shifter, multiwavelength erbium doped fiber ring source.

The calculated output power spectrum for 31 optical frequency carriers is shown in Fig. 6.

Fig. 6. Power spectrum of system with two cascaded shifters, 31 lines, channel separation: 3,125 GHz

3. EXPERIMENTAL SETUP AND RESULTS

Fig. 7. Block diagram of experimental setup

We use a narrow linewidth tunable external cavity laser as the ML, double stage non-flattened EDFA, 0,24 nm width BLF and 1,5 GHz three port AFOS. The measured value of the polarization depend diffraction efficiency (PDDE) of AFOS equals 4,2 dB. We also mapped the diffraction efficiency versus the optical wavelength and transducer frequency. The lowest AOFS first order output insertion loss was 10,4 dB. A variable coupler was used to balance ML and loop beam powers. The polarization controller PC1 allowed to adjust ML state of polarization (SOP) for the maximum diffraction efficiency, and PC2 was used to control the feedback loop polarization. For comb measurements, we constructed a flat mirror Fabry-Perot analyzer with the finesse of 200, adjustable from 20 to 300 GHz FSR and the dynamic of about 20 dB.

Fig. 8. Normalized power spectrum consisting of 11 lines, channel separation: 1,500 GHz

frequency shifter and diverse changes of SOP for different generated frequencies. The number of generated wavelengths with the flat power spectrum could be increased by shortening the loop length and PDDE equalization or by the implementation of all polarization maintaining configuration.

The configuration and mode of operation ensure single mode peaks and continuous wave operation.

We showed that broad homogenous EDF gain, the main problem of laser multiwavelength configurations, can be overcome by using a non-resonant loop configuration with a quasi steady state operation.

ACKNOWLEDGMENTS

This work was supported by the Polish Ministry of Science and Higher Education: Project No. 3 T11D 014 27.

REFERENCES

[1] S. Yamashita et al, Electron. Lett., 32 (1996), 1298-1299. [2] J. Hubner et all, Electron. Lett., 33 (1997), 139-140.

[3] A. Bellemare at all, J. Lightwave Technol., 18 (2000), 825-831. [4] S. Kim et all, Optics Comm., 190 (2001), 291-302.