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Abstract
A tunable comb source is demonstrated on a monolithically integrated photonic integrated circuit (PIC). The PIC is a two section device designed to produce a single mode tunable spectrum, and the comb is generated by gain switching one section of the two sectioned laser. The laser produces a single mode spectra with a tunable range of 1543 - 1565 nm, and combs were generated with a frequency range of 1 - 10 GHz without requiring additional optical injection to maintain the phase coherence.
Published by Optica Publishing Group under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.
1. Introduction
Optical frequency comb sources (OFCS) show significant promise in many modern day applications such as spectroscopy [1], space-based instruments [2] and high speed telecommunications [3]. OFCS generate equally spaced spectral carriers with a known phase and relation between adjacent carriers. Due to their precise and stable frequency and relative phases, they can be used in wavelength division multiplexed (WDM) communications to create coherent optical superchannels, where guard bands between neighbouring WDM signals are no longer required [4]. One method of generating frequency combs in a PIC (which is the method described in this paper) is through the use of gain switching, which involves modulating the bias current with a high powered radio frequency (RF) signal, which rapidly turns the laser on and off. This is turn would generate combs with a frequency separation between comb lines equivalent to the RF signal. Up until now, this would normally be achieved by using some method of external injection [5] or on-chip injection locking [6,7] to maintain phase coherence and allow comb lines to be generated. Less flexible forms of gain switching have been demonstrated without an additional form of injection, such as by gain switching a single mode DFB or DM laser at a frequency near to the lasers relaxation oscillation frequency (ROF) [5,8,9]. These methods results in a generated comb but the frequency separation of the comb lines is limited to near the laser’s ROF.
In this paper we focus on using a two sectioned, single cavity laser to generate frequency combs that are not limited to the laser’s ROF, without using any of the fore mentioned methods to maintain the devices phase coherence. These two sections consist of a gain section and a slotted mirror section, and both sections are biased and tuned in order to generate a single mode spectra where the side mode suppression ratio (SMSR) is greater than 30 dB at multiple wavelengths. An isolation etch exists between these two sections to allow for electrical isolation and for the two sections to be biased independently. Each section can be gain switched independently of the other, while the other section is biased and is used to maintain the phase coherence so that combs can still be generated. This single mode (SM) laser produces a single mode spectra across a tunable range of 1543-1565 nm and combs were generated with a frequency range of 1 – 10 GHz. The output spectra of the device is analysed using an ANDO AQ6317B and an APEX AP2061-A series optical spectrum analyser (OSA), which have resolutions of 0.02 nm and 0.04 pm respectively, and so the number of comb lines and their spacing can be observed.
2. Device 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 $\mu$m InGaAs cap layer, which is followed by 0.05 $\mu$m of InGaAsP, lattice matched to 1.62 $\mu$m of InP. The ridge and slot features are defined using standard lithographic techniques, with a ridge width of 2.5 $\mu$m and a height of 1.7 $\mu$m, and a slot width of 1 $\mu$m, 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 $\mu$m in length with a depth of 0.25 $\mu$m to remove the surface GaInAs and GaInAsP layers. A deep etch was also defined using standard lithography techniques which has a depth of 3$\mu$m and goes through the active region into the n-type substrate.
The laser of the PIC is made up of the gain and slotted sections. The gain section is 600 $\mu$m in length and the slotted section is made up of 7 slots with an interslot separation of 87 $\mu$m. These slots act as reflective defects along the ridge by creating regions of lower effective refractive index, effectively creating a single cavity laser that is both single mode and tunable [6], [10]. The cavity of this laser is confined using a metal etched facet (MEF) at one end of the gain section, and a cleaved facet at the opposite end of the slotted section. The MEF consists of a deep etch through the quantum wells creating an etched facet, and then is deposited with metal to create the MEF [11]. Ground signal (GS) contacts were added to both sections (as depicted in Fig. 1.) to allow for each section to be gain switched using a GS probe. The ground pad made contact with the n-type substrate via the deep etch.
3. Device characterization
3.1 DC characterization
The LIV characteristics of the device was measured via the cleaved facet using an integrating sphere to avoid loss. Current was swept across the slotted section for different bias currents across the gain section (50, 60 and 70 mA). These results are depicted in Fig. 2(a).
Fig. 2. (a) LIV measurements as current is swept through the slotted section for different bias currents through the gain section (b) example of a single mode spectrum attained at 1549.5 nm.
The threshold current of the device ranged from 25 – 45 mA depending on the bias current of the gain section. The max power measured by the integrating sphere ranged from 1.5 – 2.4 mW, again depending on the bias current of the gain section. The trend follows that as the current through the gain section increased, the device experienced a decrease in threshold current and an increase in power.
The integrating sphere was replaced by an optical fiber and the spectra of the device was initially analysed using an ANDO AQ6317B OSA. The bias current across the two sections were varied until a single mode spectra was achieved at a bias current of 60 and 100 mA through the gain and slotted sections respectively. This spectra is plotted in Fig. 2(b) and depicts a spectra with an SMSR of 34 dB at lasing wavelength of 1549.5 nm.
To fully characterise the single mode nature of the device, currents were swept across the gain and slotted sections of the device ranging from 40 – 86 mA through the gain section and 20 – 200 mA through the slotted section. At each interval the optical spectrum was recorded from the cleaved facet using an optical fiber coupled to the ANDO AQ6317B OSA. The SMSR and peak lasing wavelength was recorded and plotted in Fig. 3.
Fig. 3. SMSR (top) and lasing wavelength (bottom) of the device as the current is varied through the gain and slotted sections.
It was found that the device, made up of the gain and slotted sections, lased at a single mode (SMSR > 30 dB) and had a tunable wavelength. From the plots attained, the SM laser produced multiple single mode regions within a wavelength range of 1543 – 1565 nm. These lasing wavelengths were approximately 4.5 nm apart which is consistent with the FSR due to the interslot separation.
All these lasing wavelengths appear as the current is swept across the slotted section while the bias current of the gain is constant at approximately 70 mA. Analysis of how the spectra changes across this period is plotted in Fig. 4. Here we see how the lasing wavelengths and SMSR of the spectra change from 1543 – 1567 nm as the bias current across the slotted region varies from 25 – 250 mA.
Fig. 4. (a) Color plot showing the power of the spectra as current was swept across the slotted section. (b) SMSR and lasing wavelength of the device as the current was swept across the slotted section.
3.2 RF characterization
Once the SMSR map (Fig. 3) was developed, one of the DC probes was exchanged for a ground-signal RF probe and was used to gain switched either the gain or slotted section, while the other section is biased using a DC probe. Combs were generated by gain switching both the gain and slotted sections and are depicted in Fig. 5. Both comb examples are generated at a gain switched frequency of 2 GHz and were measured using the high resolution APEX AP2061-A series OSA. The comb in Fig. 5(a) was achieved by biasing the gain and slotted sections at 70 and 100 mA respectively, and the comb in Fig. 5(b) was achieved by biasing the gain and slotted sections at 68 and 150 mA respectively.
As mentioned in section 1, achieving phase locked frequency combs through gain switching, where the frequency separation between comb lines is equivalent to the modulation frequency, typically requires an additional form of optical injection. This has been demonstrated using external injection [5] and on-chip injection locking [6,7]. Less flexible combs have been demonstrated in [8,9] for single sectioned lasers without additional optical injection, however these combs are limited to a modulation frequency equivalent to the laser’s ROF. Fig. 5 illustrates two combs attained from this PIC. The bias currents used to attain Fig. 5(a) corresponds to an ROF of 2.896 GHz and Fig. 5(b) corresponds to an ROF of 4.013 GHz. As is well known, and observed here, the optimum modulation frequency to generate an optical comb via gain-switching is on the order of the laser’s ROF [12]. However the combs achieved here, still have stable phase coherence at frequencies away from the laser’s ROF, which isn’t in keeping with previous comb generation without additional optical injection seen in [8,9].
A full frequency response of the combs was determined by biasing the gain and slotted sections at 70 and 100 mA respectively, gain switching the gain section and adjusting the gain switched frequency. The frequency was swept from 1 – 15 GHz at intervals of 1 GHz and the results are shown in Fig. 6. The quality of the generated combs decreased as the gain switched frequency increased, which is consistent with previous on-chip gain switching techniques with additional optical injection [6,7]. However, a broader comb is achieved at 3 GHz, generating 10 comb lines within 3 dB of each other. This gain switched frequency is consistent with the devices ROF, which is measured to be 2.896 GHz at these bias currents. This is consistent with results attained in [8], where the quality of the comb could be enhanced by gain switching the device close to its ROF.
The generation of comb lines at modulation frequencies away from the lasers ROF has only been achieved on-chip through additional forms of optical injection [6,7]. The existence of such comb lines suggests that the optical gain in the non-gain switched section of the single cavity device helps to maintain the phase coherence of the device provided that power levels in the full device are not dominated by spontaneous emission during the turn-off stages, and thus combs are generated.
3.3 RF current effects on combs
The basis of gain switching is to rapidly turn the laser, or in this case the laser section, on and off repeatedly using a high powered RF signal. So it is logical that the current itself would also effect the generated comb, specifically on the amplitude of this sinusoidal current and its ability to turn the laser on and off when the minimum value of the current is too low or too high. As demonstrated in Fig. 4, the gain was biased at 70 mA and the current through the slotted section was swept from 25 – 250 mA achieving multiple single mode regions. Thus the effects of the bias current on the generated combs could be analysed by carrying out this sweep while the slotted section is gain switched.
An RF amplifier (JDS Uniphase Optical Modulator Driver H301-2310) was integrated with the experimental setup and so these current sweeps could be carried out at different amplifications of the gain switched current. The RF amplifier was powered using a standard voltage source and so the amplitude of the RF signal could be adjusted by adjusting the voltage output of the voltage source. The RF amplifier had a maximum voltage input of 8 V, and so to analyse the effects of the RF amplitude on the generated combs, the current sweeps were carried out for the RF amplifier being powered by 4 V and 8 V, to create a low and high RF amplitude.
3.3.1 RF amplifier at 4V
The RF amplifier was first powered by 4 V, the gain section was biased at 70 mA and the gain switched current through the slotted section was swept from 25 – 250 mA. The results of this are shown in Fig. 7. The resulting spectra was mostly similar to the non-gain switched sweep in Fig. 4, however, a slight reduction in SMSR was observed at bias currents < 100 mA as the lower limit of the sinusoidal current approached 0 mA. This is consistent with a known phenomena where combs can’t be generated when the lower limit of the gain switched current reaches 0 mA [9].
Fig. 7. (a) Color plot showing the power of the spectra as the gain switched current was swept across the slotted section. (b) SMSR and lasing wavelength of the device as the gain switched current was swept across the slotted section.
The corresponding combs generated at a gain switched current of 50, 115, and 225 mA are plotted in Fig. 8 and the quality of the combs degrade as the gain switched current increases. Since gain switching required the device to be continuously turned on and off, as the gain switched current increases there comes a point where gain switching is no longer possible, and so the quality of the combs degrade as the gain switched current increases.
Fig. 8. Analysis of the peak lasing wavelength while the gain switched current through the slotted section is (a) 50 mA, (b) 115 mA and (c) 225 mA.
3.3.2 RF amplifier at 8V
The analysis was carried out again, but this time the voltage powering the RF amplifier was set to 8 V to generate a high amplitude sinusoidal current. The gain section was again biased at 70 mA and the gain switched slotted section was swept from 25 – 250 mA. The results are shown in Fig. 9.
Fig. 9. (a) Color plot showing the power of the spectra as the gain switched current was swept across the slotted section. (b) SMSR and lasing wavelength of the device as the gain switched current was swept across the slotted section.
This time a large reduction in the SMSR of the spectra was observed at bias currents < 100 mA as the lower limit of the gain switched current reached 0 mA. The corresponding combs generated at a gain switched current of 50, 150, and 186 mA are plotted in Fig. 10.
Fig. 10. Analysis of the peak lasing wavelength while the RF current through the slotted section is (a) 50 mA, (b) 150 mA and (c) 186 mA.
In Fig. 10(a) what is generated cannot be considered an OFC as it contains no generated comb lines. This occurs when the modulation amplitude is high enough to fully switch off the laser and therefore the resulting pulses are built up from spontaneously emitted photons and the coherence of the pulses are lost [9]. Increasing the gain switched current to 150 mA (Fig. 10(b)), means that this high amplitude sinusoidal current is no longer resulting in the device switching off completely and so the generated comb can benefit from the high amplitude signal. This is demonstrated by the generated comb having 13 comb lines within 3 dB of each other. Increasing the current further to 186 mA, the quality of the comb again starts to degrade for increasing gain switched currents as discussed in section 3.3.1.
4. Conclusion
This paper demonstrates how gain-switched frequency combs can be generated from a two sectioned, single cavity laser, without requiring additional optical injection or gain-switching near the ROF of the laser. The laser was demonstrated to produce a single mode spectra with a tunable wavelength ranging from 1543 – 1565 nm. Combs were successfully generated by gain switching either the gain or slotted sections, while using the other section to maintain the phase coherence of the device. The quality of the combs were greatly enhanced while gain switched close to the relaxation oscillation frequency of the device. The combs were enhanced further by increasing the amplitude of the gain switched current and biasing the device at a region where the lower limit of the sinusoidal current is low enough to generate good quality combs but not low enough as to completely switch the laser off.
Funding
Science Foundation Ireland (12/RC/2276); Irish Research eLibrary.
Acknowledgment
This work was supported by Science Foundation Ireland under grant 12/RC/2276 (IPIC). Open access funding provided by Irish Research eLibrary.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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