This work investigates the optical phase locking performance of Slotted Fabry Perot (SFP) lasers and develops an integrated variable phase locked system on chip for the first time to our knowledge using these lasers. Stable phase locking is demonstrated between two SFP lasers coupled on chip via a variable gain waveguide section. The two lasers are biased differently, one just above the threshold current of the device with the other at three times this value. The coupling between the lasers can be controlled using the variable gain section which can act as a variable optical attenuator or amplifier depending on bias. Using this, the width of the stable phase locking region on chip is shown to be variable.
© 2013 Optical Society of America
Phase locking via optical coupling of semiconductor lasers has been an area of immense interest since the early 1980s  with numerous applications arising from the theoretical and experimental study of injection locked semiconductor lasers . In recent years, optical coupling has been used extensively to improve various system parameters. For example, optical injection has been used with directly modulated lasers in order to reduce the Relative Intensity Noise (RIN)  and to significantly increase the laser modulation bandwidth , while mutual coupling has been shown to dramatically decrease the optical linewidth . Much of the recent work on both unidirectional and bidirectional optical coupling in various semiconductor laser systems has focused on the non-linear dynamics observed including excitability, multistability and chaos [6–21].
Recent applications of injection locking have shown the demonstration of adaptive optical filters  and phase locked coherent laser outputs , a necessary feature of the majority of modern day modulation formats . For example in the case of Coherent Wavelength Division Multiplexing  optical coupling has commonly been proposed as a suitable technique for the generation of the required phase coherent combs.
In a typical situation, optical coupling is carried out using discrete components, where the two lasers are coupled together using free-space optics or via optical fiber. Due to the ever increasing demand placed upon current network communications, there has been a distinct shift from using discrete optical components to developing integrated photonic devices. Such integrated devices or photonic integrated circuits (PICs) have a significantly smaller device footprint along with lower power consumption and cost, thereby making them highly attractive prospects for use in modern day telecommunication networks replacing the need for expensive free space optics and discrete components.
In this work, we use an Optical Spectrum Analyzer (OSA) and an Electrical Spectrum Analyzer (ESA) to investigate the behaviour of a Slotted Fabry Perot (SFP) laser  under optical unidirectional and bidirectional coupling. The stable optical injection locking of such an SFP to an external cavity laser is investigated. Furthermore, the possibility of a PIC having two fully monolithically integrated SFPs on a single chip with a variable gain section between them operating in an optically phase locked scheme is investigated. The variable gain section of the device is biased so that light is coupled from each laser to the other. This gain section can be used to tune the coupling strength between the lasers, which provides a means of controlling the width of the phase locking region on chip. A schematic of this chip is shown in Fig. 1.
2. Device structure
The material structure for devices discussed in this work is comprised of 1550 nm laser material on an InP substrate, incorporating 5 QWs in the active region. The QWs are based on AlInGaAs with a total active region thickness of 0.4 μm. A two section single facet SFP laser was designed on this material with the required optical feedback coming from the cleaved facet on one end and the etched slotted mirror section on the other end, similar in design to those discussed in . The mirror section of the SFP laser used consisted of seven 1.7 μm deep slots with a slot width of 0.88 μm, while a 2.5 μm wide ridge waveguide provided for lateral optical confinement.
3. Optical injection locking of SFP lasers
Initially the viability of stable optical injection locking of SFP lasers is examined by considering the optical injection locking of a single SFP laser, the “Slave-SFP” (S-SFP), mounted on a temperature controller stage with access to the front facet, by a commercial external cavity tunable device, the “external master laser”, with linewidth < 100 kHz and tunable in steps of 1 pm. The external master laser output was guided in single mode fibre through port 1 of a circulator, which provides greater than 40 dB isolation, and a set of polarisation controllers and coupled to the S-SFP laser via lensed fiber. Port 3 of the circulator provides the input to the diagnostics tools of the OSA and ESA. In the experiment there are three experimentally available control parameters, namely the injected optical power from the external master laser, the S-SFP power and the frequency detuning between the external master laser and the S-SFP. The threshold current of the S-SFP, ITh, is 28 mA with an example of the spectrum shown in Fig. 2.
The external master laser wavelength is chosen to be close to the lasing wavelength of the S-SFP and is then varied across resonance with the chosen mode. For fixed external master laser and S-SFP powers, a false colour plot of the resultant electrical power spectrum of the S-SFP is shown in Fig. 3. Bright colours correspond to higher modulation powers and blue to the noise floor of our instrumentation. The dynamic range of the plot ranges from low intensity (blue) to high intensity (dark red), as can be seen in the relative intensity scale shown in the color bar.
We define the detuning as the frequency of the master minus the free-running frequency of the slave. From Fig. 3 there are three regions of distinct behaviours observed: (i) a quiet region close to zero detuning where the S-SFP is phase-locked to the external master laser; (ii) an unlocked region for both positive and negative detuning where the ESA displays at least one tone (corresponding to frequency beating far from zero detuning); (iii) a region of complex dynamics close to the negative detuning unlocking boundary (i.e. higher external master laser wavelength). While such regions of complex dynamics have proved to be of immense interest to the non-linear laser dynamics community over the past few years (again see [13–21]), this work solely focuses on the stable locking region which clearly indicates that the S-SFP has been successfully phase locked to the external master laser.
4. Integrated phase locked system
Having clearly demonstrated that stable optical injection locking of an SFP laser to an external master laser is achievable, attention is switched to the possibility of developing an integrated phase locked system on chip between two SFP lasers. To this end, two identical SFP lasers were monolithically integrated on-chip with a variable gain waveguide interconnect.
One of these lasers was biased to just above threshold. The other laser was in turn biased to typically 2.5–3.0 times threshold. At these biases, the power ratio between the two was on the order of 10:90. The higher power laser is shown on the left in Fig. 4 with two large contact pads for independently biasing the gain, IGain1, and mirror, IMirror1, sections of the SFP. The lower power laser is shown on the right of Fig. 4. At the mirror side of the high power laser, an isolated 520 μm straight waveguide section connected it to the input side of a 1 × 3 Multimode Interference device (MMI) as can also be seen in Fig. 4. The MMI was designed for splitting between three separate outputs but here is used as part of a variable gain section with only the central output being examined. The MMI for this ridge waveguide structure had a width of 12.5 μm and a corresponding length of 195 μm.
The straight waveguide connecting the higher power laser to the MMI, the MMI itself and the MMI output waveguides are all inherently lossy near 1550 nm, the proposed operational wavelength of the PIC. In order to compensate for the losses that would otherwise exist, metal contacts were deposited on the waveguide sections in order to provide current injection which can be used to control the gain for amplification/attenuation of the optical signal. The currents in these sections, IMMI and ISOA, are driven from a common current source and are collectively referred to as the variable gain section of the PIC. Applying a reverse bias on this section allowed it to act as a large photodiode, allowing for direct measurement of the optical coupling between the lasers on-chip. The PIC was tested using a custom built chuck and probe station with six probes in total required to bias the SFPs and variable gain section. Examining the picture of the device shown in Fig. 4, it is worthwhile to note that only one of the lasers on the right hand side is actually biased in this experiment and acts as the lower power laser. The SFPs and variable section were all driven individually, with lensed fibers positioned at the output facets of each SFP in order to collect the output from both lasers.
The lower power laser, referred to similarly as before as the Slave-SFP laser (S-SFP), has its mirror section biased at IMirror2 = 30 mA and its gain section biased at IGain2 = 35 mA respectively, corresponding to 1.2 times the threshold current. The higher power laser, referred to as the Master-SFP laser (M-SFP) and the S-SFP laser are ostensibly identical and as such, Fig. 2 represents the free running spectrum of both SFPs when operated at the same bias. In order to access a stable phase locked region, it was necessary to find an appropriate current biasing for the M-SFP such that it is single mode and can be varied across resonance with a chosen mode of the S-SFP by sweeping the M-SFP gain section current.
We first tested what we call “off-chip” coupling. Having chosen an appropriate current sweep for the M-SFP, the variable gain section was reverse biased thereby removing the effective on-chip coupling between the two lasers. The output of the M-SFP was then fiber-coupled to the facet of the S-SFP laser via an optical circulator and a set of polarization controllers. In this scheme, the coupling is unidirectional, with greater than 40 dB isolation between light coupled from the S-SFP to M-SFP through the circulator.
Varying the M-SFP gain section bias, IGain1, from 60 mA to 90 mA, the spectral output of the M-SFP was moved across resonance with the S-SFP. This corresponds to a wavelength detuning between the M-SFP and S-SFP of −0.08 nm to +0.16 nm. The resulting evolution of the optical spectrum of the S-SFP is shown in Fig. 5(a). The false colour plot shows a clear region of phase locking between 0.04 nm and 0.06 nm which corresponds to an M-SFP gain bias (IGain1) variation of approximately 75 to 78 mA; this is confirmed by examining the power spectrum of the S-SFP by coupling the S-SFP output directly to a 40GHz photodiode connected to an ESA. Figure 5(b) shows the resulting power spectrum of the S-SFP as it undergoes optical injection from the M-SFP. Figure 5(a) clearly indicates the main regions that one expects in phase locked systems (as previously observed in Fig. 3), namely regions of beating between the M-SFP and S-SFP indicating unlocked behaviour and the quiet central region indicating phase locked performance of the S-SFP. The intensity feature observed around 2 GHz corresponds to the relaxation oscillation frequency of the S-SFP. Having clearly demonstrated that it is possible to stably phase lock the S-SFP using the M-SFP in the “off-chip” scheme, the viability of obtaining stable phase locking in the “on-chip” scheme is examined, where the circulator is removed and the SFPs are solely coupled through the variable gain section.
The SFPs are coupled together on-chip by forward biasing the variable gain section, ISOA and IMMI to 60 mA. This effectively makes this section transparent and tuning of the bias can control the coupling strength. The mirror sections of both the M-SFP and S-SFP, IMirror1 and IMirror2, are set at 30 mA and the S-SFP gain section, IGain2, is biased at 35 mA (1.2 × ITh). The bias on the gain section of the M-SFP, IGain1, is then varied from 65 mA to 90 mA. This corresponds to a wavelength detuning range between the M-SFP and S-SFP of −0.12 nm to +0.08 nm. The ratio of laser powers means that the system is quite far from the unidirectional limit and instead displays highly asymmetric bidirectional coupling. Nonetheless, for ease we will continue to refer to the higher power laser as the M-SFP and the lower power laser as the S-SFP. From the point of view of the stability diagram, even at this ratio the behaviour is quite similar to the master-slave system as shown in . For delay-coupled quantum well (QW) lasers a commonly observed feature is chaotic synchronization  and chaos is also a common feature in QW lasers undergoing external optical feedback. In both cases the chaos results from the weak damping of the relaxation oscillations (ROs) in these devices. In our system however, the delay between the two lasers is only ∼ 0.01 ns which is significantly shorter than the period of the ROs and with such short delays it is well-known that the laser behaviour while undergoing external optical feedback is significantly more stable than in the conventional long-cavity configuration . That such a short delay-time also allows stable phase-locking in bidirectionally coupled configurations even in weakly damped lasers was demonstrated theoretically in  and experimentally in  where the coupling was face-to-face without the use of isolators. Thus one might expect to observe stable “on-chip” phase-locking in our system and indeed this is the case. Figure 6(a) and Fig. 6(b) show the optical and power spectrum of the S-SFP laser coupled to the M-SFP laser through the variable gain section on chip. Both figures demonstrate the successful stable phase locking in the on-chip optical coupling scheme, the first time to our knowledge that an integrated phase locked system has been successfully demonstrated for these SFP lasers.
The use of the variable gain section as a variable optical attenuator (VOA) or semiconductor optical amplifier (SOA) in the on-chip coupling scheme is an interesting feature of the system. Biasing of this section allows for the coupling between the lasers to be varied. This allows the width of the phase locking region to be varied, a property which makes our system attractive for studies of non-linear dynamics in coupled oscillator systems.
With the gain and mirror sections of the S-SFP both biased to 35 mA and the mirror section of the M-SFP set to 40 mA, the power spectrum of the S-SFP was investigated as the M-SFP gain section bias was swept from 90 mA to 111 mA. With the variable gain section biased at 60 mA, as shown in Fig. 7(a) a significant power coupling is achieved, and as a result, a broad stable phase locking region is observed.
On the other hand, a variable gain section bias of 30 mA reduces the coupling strength causing a narrower region of stable phase locking, as can be seen in the power spectrum in Fig. 7(b). It should be noted that successful stable phase locking on chip has been observed for various values of the biasing on all sections, in addition to those included in the text.
In conclusion, this work has shown stable injection locking of an SFP laser by both an external cavity laser and another SFP laser in an off-chip configuration. Based on the successful locking performance of the SFP laser, an integrated on-chip phase locked system was developed. The PIC is comprised of two SFP lasers, coupled via a waveguide section which can act as a variable gain section. Stable phase locking has been successfully demonstrated in this on-chip integrated system without the need for an optical isolator, for the first time to our knowledge. The variable gain section can be used as a VOA/SOA to allow for the coupling strength to be varied. By controlling the bias across this section it has been shown that the width of the stable phase locking region can be varied.
This work was supported by the Science Foundation Ireland under Grant 07/SRC/I1173 and Grant 08/CE/I1523 CTVR, The Telecommunications Research Centre.
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