We investigate the selective amplification and filtering of injection-locked slotted Fabry–Perot semiconductor lasers. Current and temperature tuning are used to selectively filter and amplify subcarriers of coherent optical combs with a selectivity of at least 10 GHz with an optical gain of up to 18 dB for filtered lines. A side mode suppression ratio in excess of 20 dB is also achieved.
© 2012 Optical Society of America
As the demand for bandwidth-intensive services continues to grow, current communication technologies are being stretched to capacity. The desire to overcome the limitations of current systems and to attain spectrally efficient solutions has led to the development of advanced modulation formats, one such example being the current focus on the use of optical orthogonal frequency division multiplexing (OFDM). The generation of optical OFDM superchannels, such as those used in coherent wavelength division multiplexing (CoWDM) , requires the generation of a coherent optical comb and the demultiplexing and multiplexing of each subcarrier so that data can be placed on each individual subcarrier. Proposed implementations of optical OFDM have been shown to require relatively few subcarriers with a separation of the order of 25 GHz or less . Coherent comb generation is currently achievable using techniques, such as mode-locked lasers or cascaded Mach–Zehnder modulators (MZMs). However, this still leaves the problem of demultiplexing the signal, while retaining coherence, so that data can be imposed on each of the subcarriers. One way in which this can be done is through the use of active filters. Distributed feedback (DFB) lasers used as active optical filters have been shown to enable amplification and filtering [3–6]. However, DFB lasers do not lend themselves to easy monolithic integration as they require multiple epitaxial regrowth steps. We propose and investigate the use of injection locking [7–10] of slotted Fabry–Perot (SFP) lasers [11–13] for the amplification of a selected subcarrier from a coherent optical comb. Easy monolithic integration is achievable using SFP lasers as they require only a single epitaxial growth step. This is important in the development of photonic integrated circuits (PICs) for integration of lasers with modulators, demultiplexers, etc. They have also been shown to be good candidates for injection locking requiring low power . In this paper we investigate the selective amplification and filtering of injection-locked SFP semiconductor lasers. Current and temperature tuning of the devices are used to selectively filter and amplify subcarriers of coherent optical combs with a selectivity of at least 10 GHz with an optical gain of up to 18 dB for filtered lines. A side mode suppression ratio (SMSR) in excess of 20 dB is also achieved.
The schematic for the setup used in this experiment is shown in Fig. 1. The slave laser used in this experiment was a single-slot SFP laser diode (SFP-LD), similar to that as described in . The device was approximately 600 μm long. The slot is positioned to create two approximately equal length sections of . The slot also electrically isolated the two sections from each other so that different drive currents could be used in each section. This laser exhibited longitudinal mode separation of 23.5 GHz with a supermodal separation of 70 GHz. The facets of the laser were not coated. Laser threshold occurred at 24 mA of drive current in both sections. The laser was mounted on a temperature-controlled mount in such a way as to allow coupling to both facets, which was achieved using lensed fibers. Positioning of the fiber was controlled using a piezoelectric actuated stage on the input side and a manual micrometer actuated stage on the output side. A circulator was used on the input side to provide optical feedback for the piezoelectric controller allowing for automated coupling. A tunable laser source (TLS) was used as a master laser, which was modulated using a MZM. This allowed the creation of a coherent optical comb, as shown in Fig. 2. Throughout the experiments, the MZM bias was biased to operate at quadrature so that the power in the original carrier was unaffected by the RF input signal. This bias enabled equalization of the peak powers of the carrier and the first-order sidebands. The output signal was directly analyzed on an optical spectrum analyzer (OSA). The two sections of the laser were driven electrically with a current source on each section. Selecting different drive currents in each of the section allowed tuning of the output spectrum of the laser. Temperature was maintained at ambient room temperature when the temperature was not being actively altered as part of a temperature sweep.
3. Results and Discussion
A. Wavelength Sweep
To find a baseline performance of the laser as a selective amplifier and filter, single carrier injection was used. This was achieved by switching off the microwave source driving the MZM while maintaining the DC bias. The wavelength of the master laser was then swept across resonance with one of the free running laser modes of the SFP laser. An OSA trace was saved for each injection wavelength, and a two-dimensional intensity plot was generated as a graphical representation of the evolution of the slave laser’s optical output during a wavelength sweep. An example of a wavelength sweep can be seen in Fig. 3. Here the injection power was . This injection power was measured by reverse biasing the laser device and using it as a photodiode. The wavelength was swept from 1563.0 to 1563.4 nm, and the drive current was set at 30 mA in both sections of the laser. A single locking event occurs during the sweep centered at 1563.19 nm. An SMSR greater than 20 dB was maintained for over 6 GHz. Another event occurs at an injection wavelength of 1563.39 nm. This was an unwanted interaction between the master TLS and the next longitudinal mode of the laser. This is discussed in more detail below for coherent comb injection.
The wavelength sweep was then repeated with the RF input to the MZM switched on. The RF frequency was set to 10 GHz creating 10 GHz sidebands on each side of the carrier. The drive currents and injection power were the same as for the nonmodulated sweep. The resulting plot can be seen in Fig. 4. All three different input wavelengths locked the SFP laser in turn as the external cavity laser output wavelength was swept through the wavelength range. Injection locking instances can be seen at injected wavelengths of 1563.11, 1563.19, and 1563.28 nm. An SMSR of greater than 20 dB is maintained for widths of 3.17, 2.17, and 0.94 GHz.
The high output power of the injection-locked mode relative to the unfiltered optical lines, coupled with the resolution capability of the OSA, resulted in a blurring of the data. This prevented the sidebands A and C being detected when the carrier B was locked and also prevented the carrier, B, from being detected when the slave was locked to either of the modulated sidebands. However, the wavelength spacing between the modulated sidebands was sufficient so that, if either was locked to the slave, the other sideband was observed and the SMSR can be recorded. Figure 5 shows the OSA data, demonstrating stable single-mode operation with an SMSR in excess of 20 dB.
The modulation frequency used in the experiment was 10 GHz; this creates a frequency spacing of 20 GHz between the modulated side bands, which is close to the slave laser’s natural mode spacing of 23.5 GHz. In Fig. 4 at 1563.28 nm, when signal A (from Fig. 2) is in resonance with the target mode of the slave laser, one can also see that signal C is also nearly in resonance with the next mode of the slave. This leads to an unintended amplification of signal C (the sideband) as seen in the red trace of Fig. 5. This leads to a reduction in the SMSR. Although the desired wavelength A is transmitted and experiences gain, wavelength C is also transmitted and experiences some gain, which reduces the SMSR and thus the effective width of the filter. This can be seen, in the wavelength sweep in Fig. 4, for the width of the region where the SMSR of the filter is greater than 20 dB. The effective filter width is 3 GHz for the first locking instance at 1563.11 nm, while the third locking instance at 1563.28 has an effective width of 0.94 GHz. Ideally the slave laser’s mode spacing should be optimized to ensure that only one resonant mode of the laser corresponds to any subcarrier of a comb source.
B. Temperature Sweep
To use the SFP-LD as a tunable filter, the resonant condition of the laser needs to be changed. This can be achieved by temperature tuning the SFP laser. By controlling the temperature of the chuck on which the laser was mounted, it was possible to vary the SFP laser’s resonant condition. The temperature sweep resulted in a wavelength change of approximately . Again, to set a baseline measurement for the laser performance, the temperature sweep was first done with a nonmodulated signal shown in Fig. 6 so that a single wavelength was injected. The drive currents and injection power were the same as before. In this particular example the temperature was altered from 20°C to 22°C in 0.1°C increments. Injection locking of the slave laser occurred at temperature of 20.9°C. An SMSR of greater than 20 dB was maintained for a frequency span of 0.32 GHz. This width is considerably smaller than that of the wavelength sweep as the injection wavelength is farther from the gain peak.
The RF input to the MZM was again switched on and the temperature sweep was repeated. The resulting data are shown in Fig. 7, clearly showing three different injection locking regions over the course of the sweep at temperatures of 20.1°C, 20.9°C, and 21.7°C. At each temperature point, a different wavelength was filtered and amplified. Figure 8 shows a sample SFP-LD output from each of these injection locking regions. A minimum SMSR of 23 dB was achieved while the master wavelengths of 1563.29, 1563.38, and 1563.46 nm were maintained at the output, matching the 10 GHz separation between the injected comb’s subcarriers. An SMSR in excess of 20 dB was maintained for widths of 0.06, 0.14, and 0.16 GHz. The maximum gain for any of the filtered subcarriers was approximately 17 dB.
C. Current Sweep
The resonant wavelength of the SFP-LD was also tuned by changing the injection current in the different sections of the slave laser. The drive currents in both sections of the lasers were changed simultaneously and by the same increment. Current tuning resulted in a wavelength tuning rate of . The sweep was first performed for the nonmodulated input, with the result shown in Fig. 9.
The current was swept from 29 to 39 mA in increments of 0.1 mA. For the case of a single injection carrier, locking occurs at a current of 33.61 mA. An SMSR of greater than 20 dB is maintained for a width of 0.62 GHz. The master laser power was again set at , and the temperature was maintained at ambient room temperature. The RF input to the MZM was again switched on to generate the coherent comb. All parameters were kept as for the single signal carrier injection case above. The sweep was repeated resulting in the data shown in Fig. 10. Three separate locking instances are shown at operating currents of 30.5, 33.61, and 36.75 mA, where in each case a separate subcarrier was filtered at each current bias. An SMSR of greater than 20 dB is maintained for each of the locking instances with widths of 0.67, 2.84, and 0.91 GHz. Figure 11 shows the OSA traces for each locking instance at the currents listed above. Minimum SMSR achieved was 21 dB with a maximum gain of 18 dB.
In this paper, we have demonstrated the selective amplification and filtering of subcarriers of a coherent comb source using injection locking of an SFP semiconductor laser. Tunability of the filter was achieved through varying the laser injection current and temperature to select the desired optical frequency from a frequency comb. The technique presented here has demonstrated a frequency selectivity of 10 GHz, with a suppression of unwanted wavelengths in excess of 20 dB, for each of the tuning methods. The maximal gain achieved for a selected subcarrier was of the order of 18 dB. This work has demonstrated the capability of using injection-locked SFP lasers as a wavelength-selective, tunable optical amplifier that is useful in the development of PICs for coherent communications. This is particularly useful for CoWDM where the ability to filter individual subcarriers while maintaining coherence is required.
This work was supported by Science Foundation Ireland under grant SFI 10/CE/I1853 CTVR II.
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