Gain Switched Frequency Comb Enhancement Using Monolithically Integrated Mutually Coupled Lasers

A tunable comb source is demonstrated through gain switching a three sectioned photonic integrated circuit (PIC). The PIC consists of two mutually coupled lasers, integrated on an InP epistructure. One of these lasers is a tunable, two-section, single mode laser and its tunability is demonstrated by varying the bias current across the two sections, producing distinct wavelengths with a tunable range of 1535–1560 nm. The second laser is a simple Fabry-Pérot cavity laser which can be phase-locked with the single mode laser. Frequency combs were produced by phase-locking the two lasers and then gain switching the Fabry-Pérot laser by applying a high power radio frequency (RF) signal. In this letter we explore the effects that the biasing symmetry between the two coupled lasers has on the resulting gain switched comb. We demonstrate that a mutually coupled system results in enhanced gain switched frequency combs with a broader bandwidth and higher power compared to the asymmetrically biased coupling.


I. INTRODUCTION
O PTICAL frequency comb sources (OFCS) show significant promise in many modern day applications such as spectroscopy [1] and high speed telecommunications [2]. OFCS generate equally spaced spectral carriers with a fixed phase relation between adjacent carriers. One method of generating frequency combs in a PIC (which is the method described in this letter) is through the use of gain switching, which involves modulating the bias current with a high powered RF signal, which rapidly turns the laser on and off. Very often this is employed in conjunction with some method of external injection [3] or on-chip injection locking [4], [5] to maintain phase coherence and allow comb lines to be generated.
Coupled cavity semiconductor lasers were modelled extensively in the 80s and 90s by G.P. Agrawal  a significant reduction in the chirp of the coupled cavity system under optimum biasing conditions [6]. On-chip coupling of monolithically integrated single mode lasers has been examined experimentally in [7] and [8] with very different configurations and design types where, despite the lack of on-chip isolation, mutual phase locking was achieved. In both cases, the biasing of the lasers was highly asymmetric where one laser (usually known as the secondary laser) was biased at threshold current and the other laser (the primary) was biased above threshold. Stable phase locking was achieved in both cases as the secondary was optically phase locked to the emission of the primary laser, thereby approximating a primary-secondary configuration. Alternative coupling techniques, whereby this biasing asymmetry is discarded while maintaining stable phase locking, have been demonstrated in [9], [10], and [11]. In each of these cases, both coupled lasers were biased well above threshold current, therefore operating in a mutually coupled system. Further experimental analysis into the benefits of the on-chip coupling of gain-switched lasers, particularly in relation to the laser's linewidth and phase noise, were investigated in [4] and [12] respectively. These examples use a similar fabrication material, process and design as the example described in this letter, and so their results are assumed to be equivalent to the linewidth and phase noise of the device in this work. These two examples concluded that the linewidth of a single longitudinal mode (SLM) laser increases from 200 kHz before gain switching to 1 MHz with a five time increase in phase noise after the SLM laser is gain switched. However, when the SLM laser achieves stable phase locking with a coupled Fabry-Pérot (FP), the linewidth remains at 350 kHz before and after gain switching with no major increase in phase noise.
In this work, we couple a SLM laser and a FP on-chip and demonstrate stable phase locking through the asymmetric primary-secondary coupling and the non-asymmetric mutual coupling configurations. Furthermore, we show that by gain-switching the FP laser, stable combs are produced and that the mutual coupling enhances the resulting OFCs by broadening the bandwidth of the generated combs compared to primary-secondary methods [4], [5]. The output of the device is analysed using an ANDO AQ6317B and an APEX AP2061-A series optical spectrum analyser (OSA), which have a resolution of 0.02 nm and 0.04 pm respectively, and so the number of comb lines and their spacing can be observed.

II. THE DESIGN
This device was fabricated with commercially available lasing material designed for the emission at 1550 nm, purchased from the company IQE. This lasing material consists of 5 compressively strained 6 nm wide AlGaInAs quantum wells on an n-doped InP substrate. The upper p-doped cladding consists of a 0.2 µm InGaAs cap layer, which is followed by 0.05 µm of InGaAsP, lattice matched to 1.62 µm of InP. The ridge and slot features are defined using standard lithographic techniques, with a ridge width of 2.5 µm and a height of 1.7 µm, and a slot width of 1 µm, with the ridge etch stopping above the quantum wells. An isolation slot exists between the two sections of the device for electrical isolation. This is an etch in the ridge of the laser 15 µm in length with a depth of 0.25 µm to remove the surface GaInAs and GaInAsP layers. A deep etch was also defined using standard lithography techniques which has a depth of 3µm and goes through the active region into the n-type substrate.
The PIC consists of a two section SLM laser and a single section FP laser. The SLM laser has a gain section 600 µm in length and a slotted section consisting of 7 slots with an interslot separation of 87 µm. These slots act as reflective defects along the ridge by creating regions of lower effective refractive index, effectively creating a laser that is both single mode and tunable [4], [5], [13]. The last slot in the SLM laser is deep-etched thereby forming an etched facet, and is free of metal to allow for electrical isolation. The cavity of the SLM laser is confined using this deep etched facet at one end of the slotted section, and a metal etched facet (MEF) at the opposite end of the gain section. The MEF consists of a deep etch through the quantum wells creating an etched facet, which is then deposited with metal to create the MEF [14]. The FP laser is 420 µm in length and the cavity is confined by the deep etched slot of the SLM laser, and a cleaved facet. A Ground Signal (GS) contact is added to the FP (as depicted in Fig 1.) to allow for gain switching using a GS probe. The ground pad makes contact with the n-type substrate via the deep etch.

A. DC Characterisation
The LIV characteristics of the two lasers were measured independently using fiber coupling through the cleaved facet. Initially the FP laser was biased and current was swept from 0 -100 mA, the corresponding voltage and optical power were measured and plotted in Fig 2(a). Similarly the LIV characteristics of the SLM laser were measured by sweeping the current through the gain section for different bias currents through the slotted section (Fig 2(b)). This time the FP section was biased at 10 mA, which is high enough to make the section passive but not high enough to make the section lase, and so the SLM laser could be measured independently through the same facet as the FP.
The threshold current of the FP laser was measured to be 25 mA and the threshold current of the SLM laser ranged from 12 -35 mA depending on the bias current of the slotted section. The plotted LIV characteristics of the SLM laser are non-linear as a result of changes in the emission mode. These changes are demonstrated in Fig 3(a) where a colour plot illustrates the measured spectra of the SLM laser as current is swept across the gain section for a bias current of 90 mA across the slotted section. The corresponding SMSR and lasing wavelength are plotted in Fig 3(b).
The spectra of the two lasers were also measured through fiber coupling using a high resolution APEX OSA. The spectrum of the FP laser attained at a bias current of 50 mA is plotted in Fig 4(a). A single mode spectrum attained from the SLM laser by biasing the gain and slotted sections at 75 and 100 mA respectively is plotted in Fig 4(b).

B. Primary-Secondary Coupling
As mentioned in Section I, one method of on chip coupling involved biasing the integrated lasers asymmetrically in a strongly coupled primary-secondary configuration. A single mode spectrum was achieved by biasing the gain, slotted and FP sections at 150, 40 and 25 mA respectively. The resulting spectrum is plotted in Fig 5(a) and has a lasing wavelength and SMSR of 1556.4 nm and 50 dB. In order to gain switch the laser, the probe biasing the FP section was replaced and biased with a ground signal (GS) probe. The current supplied to this GS probe was provided by a Keithley 2400 series that  was modulated with a frequency generator via a bias tee. This resulted in a sinusoidal current with a frequency equivalent to that determined by the frequency generator and would rapidly turn the FP laser on and off (gain switching). An example of a generated comb at a gain switched frequency of 6.8 GHz is plotted in Fig 5(b).
The frequency separation between the adjacent comb lines is equivalent to the frequency of the gain switched signal, which in this case is 6.8 GHz. In terms of the quality of a frequency comb, the comb attained here is quite poor as the comb lines are few and there is a large power difference between adjacent comb lines. The quality of the comb is largely dependent on the frequency of the gain switched current, with optimum combs known to be achieved at a frequency around the laser's relaxation oscillation frequency (ROF) [15], which in this case was measured to be 3.3 GHz.

C. Mutual Coupling
The bias in the slotted section was set to 20 mA and current was swept across the gain section and the FP. This takes the mutually coupled system out of the asymmetric bias regime examined in [7] and [8]. At each interval the optical spectrum was recorded from the cleaved facet using an optical fibre coupled to an ANDO AQ6317B OSA. The SMSR and peak lasing wavelength were recorded and plotted in Fig 6.  Fig 6 demonstrates how the multiple independent sections of the PIC allow for the emission wavelengths to be more effectively tuned compared to solitary sectioned devices. This coupled system generates six different single mode wavelengths ranging from 1535 -1560 nm. The gain, slotted and FP sections were biased at 80, 20 and 50 mA respectively and the resulting single mode spectrum was plotted in Fig 7(a). Analysing the spectra further with the higher resolution AP2061-A series OSA, this mutually coupled system resulted in an increase in power and SMSR from -25 dBm and 50 dB (Fig 4(b)) to -10 dBm and 60 dB respectively (Fig 7(a)).
Once this single mode spectra was achieved, the FP section was once again biased using a GS probe and the device was gain switched. The corresponding comb is plotted in   Fig 7(b) at a frequency of 6.8 GHz with the ROF of the FP laser measured to be 3.6 GHz. What results is an enhanced frequency comb with a broader bandwidth and higher power compared to the frequency comb generated using the primarysecondary configuration demonstrated in Fig 5. Qualitatively similar results can also be found at different lasing wavelengths that could generated from the mutually coupled system as demonstrated in Fig 6.

IV. CONCLUSION
This letter demonstrates how a three section PIC can be used to generate and enhance gain switched combs. The effects of biasing symmetry between two coupled lasers, on the resulting gain switched combs are investigated and this letter demonstrates that asymmetric bias coupling can be discarded in favour of a mutually coupled system. Here we demonstrate that combs generated from a mutually coupled system results in not only maintaining stable phase locking with an improved power and SMSR, but the generated combs are enhanced to be detectable across a wider spectral range.