Multiwavelength and Multifunctional Erbium-doped Fiber Laser Based on Arrayed Waveguide Grating C. Barros, H. G. Rosa and E.A. de Souza Laboratório de Fotônica - Universidade Presbiteriana Mackenzie Rua da Consolação, 896 – São Paulo / SP – Brazil
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Abstract - We demonstrated a multiwavelength and multifunctional ring cavity laser based on integration of two Athermal Arrayed Wavelength Gratings (AWG). We demonstrated that both AWG are reversible, and making laser capable to operate simultaneously in continuous wave in one channel and passively mode-locked by single-walled carbon nanotubes in another channel. Keywords: Arrayed waveguide gratings, multiwavelength laser, multifunctional laser, carbon nanotubes, passive mode-locking.
I.
INTRODUCTION
Multiwavelength lasers are very attractive for WDM systems. This kind of laser operates with many channels from different wavelengths each one, at a specific repetition rate, and can be used to increase transmission’s capacity, by increasing the numbers of channels simultaneously generated and transmitted in just one single mode optical fiber [1]. Multiwavelength, and possibly multifunctional lasers, can be built using integrated waveguide gratings [2], where their geometries defines division and combination of channels, spacing between channels, and quality (relative power levels, for example) of each channel [3, 4]. Arrayed Waveguides Gratings (AWG) are effective demultiplexing and multiplexing devices that can be used to generated multiwavelength laser sources. They are especially attractive for WDM systems due to their low insertion loss. Normally, we need a accurate temperature control of this dispositive, since they show a sensitivity about 0.01 nm/°C [5]. In this work, we built a multiwavelength laser by connecting two AWGs inside a ring laser cavity. This laser configuration shows simplicity and a great flexibility to operate as a multiwavelength and also as a multifunctional laser, since it needs just one amplifier for all wavelengths. It can operate, simultaneously, as a CW, an active mode-locked, and a passive mode-locked. An advantage of this scheme is the accurate separation between channels, without individual sources for each channel, allowing the use of all 40 channels for any functions the system needs, monitored in a single output port. Channels can be independently modulated, at different rates, which is an advantage over precise analog control of DBR lasers [6].
II.
EXPERIMENTAL SETUP
Figure 1 shows the sketch of ring laser cavity. It consists of an Erbium-doped fiber, a WDM to couple the pump at 1480 nm, an isolator and a polarization controller. To analyze the output signal we utilized an output coupler of 24%. To generate the channels, we inserted into cavity two AWGs in way to interconnect channel by channel, as shows in fig. 1. Note, this configuration allow us to control the function of each channel. The configuration described above, from the best of our knowledge, had never been used in multiwavelength lasers before. Most of digitally tunable lasers that interconnect two multiplexers couples their output or input ports, then light is extracted from this common port between the two devices, what can optimize or not the output power [7]. However they cannot insert functions on channels. The waveguide gratings utilized here are FITEL models PS701, with a fiber length of approximately 3.5 m each one, with 40 channels spaced by 100 GHz and with an insertion loss of 2.0 to 3.0dB.
Fig.1. Experimental apparatus of a multifunctional Erbium-doped fiber laser in a ring configuration with two AWG generating 40 wavelength.
III.
RESULTS
The waveguide gratings demultiplex the signal and then multiplex it showing reversibility so we can measure all 40 channels output spectrum, well defined and equally spaced by 100 GHz (~ 0.8 nm). All channel are measured at the same time, connecting a single output fiber to an optical spectrum
This work was sponsored by MACKPESQUISA and FAPESP.
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analyzer. The performance of all channels is shown in figure 2.
Fig. 2. Average output power as a function of total pump power for all 40 wavelength in CW operation.
We can see in figure 2 that there is no difference between the laser characterization when it operates with a single one or with 40 channels. From our results, our laser has a efficiency of 2%. The output spectrum of the laser is shown in figure 3. With a pump power of 40 mW (before threshold) we can observe a 40 channels amplified spontaneous emission. The Free Spectral Range (FSR) is 0.79 nm. The first channel emits in 1531.92 nm and the last one emits in 1564.01 nm.
power, with environmental conditions such as temperature and humidity.
Fig. 4. Emission from the multifunctional EDFL showing flutuations for a pump of 200 mW.
Here we analyzed physical aspects of optical signal and not technical aspects of arrayed waveguide gratings, but in [9] author explained that loss can be minimized by elimination of unwanted modes since a periodic array works as a mode converter. To demonstrate the laser multifunctionality we utilized all channels weakly coupled, and the channel of interest strongly coupled. For example, interconnecting channels 32 of two AWGs and inserting the single walled carbon nanotubes (SWCNT) between them, as show figure 5, we obtained a passive mode-locking laser with approximately 6.7 MHz of repetition rate in a cavity length of ~30 m (figure 6).
Figura 5: SWCNT to generate passive mode-locking regime between channels 32 of AWGs. Note this regime is observed at output cavity laser. Fig. 3. Laser operating with 40 different wavelengths in CW regime.
The signal to noise ratio of this spectrum is about 12 dB. Although all wavelength channels are defined, they have different gains that depend on Erbium-doped fiber linewidth [8]. Figure 4 shows emission of 40 channels with laser at 9 different wavelengths for a pump of 200 mW. We observed that emission laser in each channel varies with input
Carbon nanotubes control the total loss of the cavity, absorbing lower intensities of light, and saturating its absorption to higher intensities. This process leads to pulse formation. In this work we used SWCNT polymer thin film to passive mode-lock our laser. This sample was constructed and characterized by the method described in [10].
2009 SBMO/IEEE MTT-S International Microwave & Optoelectronics Conference (IMOC 2009)
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IV.
CONCLUSION
In this work we built a multiwavelength and multifunctional ring cavity laser, based on Athermal Arrayed Wavelength Gratings. We demonstrated simultaneous operation of CW and passive mode-locking by SWCNT. In future works, other regime of operation can also be added and increasing the laser functionality.
REFERENCES [1]
Figure 6: channel 32 mode-locked by SWCNT.
After obtaining a mode-locked (ML) channel, we coupled channels 21 and could obtain the simultaneous emission of channels 21 and 32, as shows figure 7. The channel 21 operates at CW regime and channel 32 at passive modelocking regime. The CW Channel emits at 1547.8 nm, and the mode-locked channel emits at 1556.5 nm.
CW
Mode-locked
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Fig. 7. Two wavelength laser operating in two different regimes, CW and passive mode-locking by a SWCNT. .
2009 SBMO/IEEE MTT-S International Microwave & Optoelectronics Conference (IMOC 2009)
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