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Abstract
In this paper, a monolithically integrated ∼1.55 µm semiconductor laser in the fourth harmonic colliding pulse mode locking configuration is reported. This device was developed within a multi-project wafer run at an InP-based active-passive generic foundry. The 1.66-mm Fabry-Pérot cavity is formed with two on-chip reflector building blocks rather than cleaved facets. In the cavity, three absorber sections symmetrically divide the cavity in four gain segments. This laser diode is able to emit 100-GHz pulse trains with 500-fs pulse duration as well as two-tone emissions with a frequency separation of 2.7 THz. The dependence of the spectral behavior on the forward bias current for gain sections and the reverse bias voltage for absorber sections are experimentally demonstrated.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Passive mode locking (PML) using III–V semiconductor lasers in photonic integrated circuits (PIC) generating optical frequency combs (OFC) and ultrafast pulse trains have attracted attention in recent years, due to its chip-scaled compactness and simplicity of DC operation. Especially, PML at repetition rates (RR) in millimeter-wave (mmW, 30–300 GHz) and terahertz (THz, 0.1–10 THz) ranges have been considered promising in communications, spectroscopy, and sensing [1–3]. However, in most cases an extremely high RR (>100 GHz) of a mode-locked lasers (MLL) is corresponding to only a few hundreds of µm cavity length (<400 µm). And such a short gain medium inevitably imposes a limitation on the output power due to saturation effects and insufficient gain to compensate for resonator losses. For this reason, harmonic mode locking (HML) schemes have been investigated to produce multiple pulses per round trip in a sufficiently long cavity, thus pushing the RR beyond the low-GHz fundamental cavity round trip frequency [4].
HML is achieved by means of colliding pulse ML [5], coupled cavity ML [6], and methods based on the wavelength selectivity of distributed Bragg reflector (DBR) grating [7]. The same research group also observed that higher-order harmonic ML occurs at high bias level [8]. In colliding pulse ML, one saturable absorber is placed at the midpoint of cavity, where two counter propagating pulses circulate and collide, producing a train of pulses at a RR that is twice the fundamental round trip frequency. Evolving from colliding pulse ML, multiple colliding pulse ML features multiple saturable absorbers concatenated with gain sections, for RR multiplication >2 as has been extensively investigated [9–11].
Edge facets have been used as mirrors to realize a multiple colliding pulse ML cavity, as they are relatively easy to make by cleaving the chips, unlike using DBRs as mirrors that requires a costly sub-wavelength fabrication process. However, for design reproducibility, high precision cleaving is seriously needed which hinders the practicability. Besides, laser signals are emitted directly out of the chips through cleaved facets, making on-chip signal processing impossible.
Recently, a new class of on-chip broadband reflector based on the multimode interference (MMI) principle has been proposed [12], and demonstrated its wide applicability [13–17]. Such multimode interference reflector (MIRs) are so simple to create in lithography with greater fabrication tolerance to replace DBRs and cleaved facets. Furthermore, they can be placed anywhere on a chip, and the transmitted light is manipulable on chip to fulfill more functionalities [18].
MIRs have been therefore adopted in on-chip colliding pulse mode locking [19,20], but none of these works has reached balanced and higher (>2) order operation for pulse amplification and reshpaing [21]. In this paper, we report a monolithically integrated MIR-based fourth harmonic colliding pulse MLL in a symmetric configuration. In addition to generating the fourth harmonic OFC and ultrafast pulse trains, even higher-order harmonic operations are observed at aforementioned higher bias levels [8]. It is worth highlighting again that this terahertz-rate emission based on higher-order harmonic generation does not require any grating-based DFB or DBR component [22–24]. This laser chip was developed within a multi-project wafer (MPW) run in a generic foundry platform [25,26], where the MIRs, gain sections, and saturable absorbers were deployed as standard building blocks called from the open-access process design kit (PDK) for cost-effective prototyping.
2. Device description
Figure 1(a) shows that the device under investigation is a multi-section structure. The laser cavity is formed with a pair of MIRs, in which there are four gain sections separated by three saturable absorber sections, placed at every quarter of the cavity length. The on-chip reflectors MIRs, define a cavity length (L) of ∼1.66 mm, corresponding to a cavity round trip frequency of ∼25 GHz. Every quarter (L/4) is ∼0.42 mm long.
Fig. 1 (a) The total cavity length L ∼ 1.66 mm is separated with three saturable absorbers that equally divide the cavity in four, L/4 ∼ 0.42 mm. (b) The Fabry-Pérot cavity defined by two multimode interference reflectors (MIR, green) has longer SOAs as gain sections (SOA) and shorter SOAs as saturable absorbers (SA). The extended connection of active components (green) is carried out with DC routing (red). The passive waveguides (blue) also contribute to the cavity length and direct the light to the waveguide outputs at facet.
Gain sections and absorber sections are realized using the same active type building block, semiconductor optical amplifier (SOA). When a SOA is forward-biased it behaves as a gain section. And when reverse-biased, it serves as a saturable absorber (SA), which absorbs light at low intensity and transmits light at high intensity. In the design, all three SAs are as short as 20 µm. The two inner gain sections, which are much longer (∼350 µm) than the two outer gain sections ∼180 µm. Each of the gain and the SA sections has a metal deposition layer. The metal pads (green) are contiguously connected through metal routes (red) to their symmetric reflections with respect to the central SA, as shown in Fig. 1(b). Electrical isolation sections were inserted between every two adjacent SOAs to avoid unfavorable current flows.
At each end of the cavity, one 2-port MIR terminates the straight waveguide at one port defining the overall Fabry-Pérot cavity length, but also guides the resonating light out of the cavity through the other port. The two MIRs are ∼55-µm long, having two ports with ~40% transmissivity and ∼40% reflectivity, based on the multimode interference principle and internal etched reflection walls [12]. For retaining the quarter geometry, ~75-µm passive deeply etched waveguides were used between the outer gain sections and the 2-port-MIRs. Also, active-passive transition sections were used to couple light through butt-joint regrowths.
As there are two MIRs, the light exits from the cavity from both sides. The light from the left MIR travels along a 90° arc waveguide prior to a straight-arc-straight connector waveguide, and heads towards a waveguide output at facet. The shallowly etched waveguide output is 7° upwards angled and tapered to minimize back-reflections at facet. A lensed fiber couples the emitted light from the microchip to one of the measuring instruments.
To control the integrated device, all SOAs are electrically connected with external current or voltage sources. Four DC probe are physically contacted on the four DC pad extensions for outer gain sections, outer absorber sections, inner gain sections, and inner absorber section, respectively, as shown in Fig. 2(a). Consequently, the voltage of all the three SAs is set through the two upper pads which are jointly connected to an external voltage source (VSA). Similarly, the two lower pads are fed with one common source from a laser diode driver that delivers a current (ISOA) shared by all the four gain sections in parallel. The backside of the substrate is grounded, and TEC-controlled at 16° with the same laser diode driver.
Fig. 2 (a) One DC probe needle electrically contacts each of the four DC pads. (b) The device under test is controlled with the voltage source VSA and the current source ISOA. The output pulse train is evaluated using the optical power meter (OPM), the optical spectrum analyzer (OSA), and the autocorrelator (AC). The polarization controller (PC) and the EDFA must be used to satisfy the polarization state and sensitivity of AC.
Figure 2(b) illustrates that the two input parameters, reverse-bias voltage VSA and forward-bias current ISOA determine the operation state of the MLL PIC under test. The output signal from PIC is analyzed with the measuring instruments. From the angled waveguide output, the fiber-coupled optical signal is sent to either an optical power meter (OPM), an optical spectrum analyzer (OSA), or an intensity autocorrelator (AC) following an erbium-doped fiber amplifier (EDFA) linked with a polarization controller (PC). The OPM provides the intensity information, essential in evaluating output power level of the MLL as well as the alignment of fiber coupling. The OSA visualizes and records an optical spectrum with a resolution of 0.03 nm. The EDFA boosts the optical power to ∼5 dBm to meet the sensitivity requirement of the AC. The PC is used to optimize the polarization dependence.
3. Characterization results
Figure 3(a) presents the L-I-V characteristics. The reverse bias voltage VSA is either switched off (= 0 V) or biased with 1 V, and the forward bias gain current ISOA is varied from 0 to 200 mA in a step of 10 mA to present the dependence of voltage of the gain sections (green) and the fiber-coupled power (red and blue) on the gain current ISOA. The threshold current is around 40 mA, and the maximum achieved average power in fiber is ∼660 µW for VSA = 0 V. For VSA = 1 V, the threshold current is slightly larger than 40 mA, and the maximum fiber-coupled power is ∼660 µW. Even though these lie in the typical range of the generic technology [17,27,28], but the output power can be further amplified by using on-chip booster amplifier to push the output power up to a few mW [20].
Fig. 3 (a) Light-current-voltage (L-I-V) characteristics of the MLL PIC under test. By sweeping the forward bias gain current ISOA from 0 to 200 mA, the voltage of the parallel gain sections jumps from near zero to 1 V and then smoothly increases up to 2 V for VSA = 0 V (green). The average output power coupled by the lensed fiber also increases, exhibiting a threshold of ∼40 mA and a maximum power of ~600 µW for VSA = 1 V (red) and ~660 µW for VSA = 0 V (blue). (b) Amplified spontaneous emission (ASE) spectrum of a 400-µm-SOA with bias current from 0 mA to 100 mA.
The output power follows a monotonic trend as ISOA increases, but not in a linear manner. It can be seen that in the first half of the trace the L-I curve exhibits a linear behavior with slope efficiency of ∼7.5 mW/A when ISOA = 40 – 100 mA. And the curve becomes less steep with slope efficiency of ∼1.5 mW/A when ISOA = 100 – 200 mA. The turning point on the curve might indicate that the laser operates differently in these two regions. The optical spectra in these different operations are experimentally shown in next section. In the whole investigated range that ISOA = 0 – 200 mA, the laser has not reached gain saturation, as no decline is observed on the curve. The upper limit of ISOA is set to 200 mA to avoid damage. The voltage of parallel gain sections lies in the typical range as well. The voltage suddenly rises to ~1 V and afterwards gradually increases to 2 V. Figure 3(b) shows the amplified spontaneous emission (ASE) spectrum of a 400-µm SOA with bias current of 0 – 100 mA. The SOA has a smooth gain profile spanning over > 100 nm. With bias current of 70 mA and 80 mA, the 1550 nm wavelength region has a broad gain profile > 10 dB.
For evaluating the spectral performance on the wavelength domain, one of the exemplary optical spectra is shown in Fig. 4(a), under ISOA = 99 mA and VSA = 1 V. In the spectrum, an OFC near 1570 nm is presented with seven comb lines and the mode spacing in between is approximately 0.8 nm, equivalent to 100 GHz in frequency. The seven lasing modes constituting the OFC exhibit a suppression ratio of ∼30 dB, with respect to the level of the suppressed modes that are associated with the fundamental cavity round trip frequency of 25 GHz. From 1570 to 1575 nm, three modes in every four modes are suppressed, and only one mode in every four modes is excited. This spectral selectivity agrees with the harmonic order of four in the cavity design. A Gaussian-based curve is applied to fit the peaks in the emission spectrum. However, the central mode of the Gaussian shape is not the highest mode. The fitting curve might not perfectly reflect the characteristic of the frequency comb. Also, the number of comb lines is limited, which fails to give an reliable estimated 3-dB bandwidth through the Gaussian fitting curve. In Fig. 4(b), the intensity autocorrelation trace under the same bias condition is shown within a span of 5 ps to evaluate the time-domain performance. The autocorrelation intensities are normalized to scale from 0 to 1. Fitted to a Gaussian function, the FWHM (full width at half maximum) of the autocorrelation profile is about 2.18 ps. The deconvoluted pulse duration is then estimated to be 1.54 ps assuming a Gaussian shape.
Fig. 4 (a) Optical spectrum with ISOA = 99 mA and VSA = 1 V. (b) Autocorrelation trace ISOA = 99 mA and VSA = 1 V.
Similarly, the optical spectrum and the autocorrelation trace under another bias condition, ISOA = 99 mA and VSA = 2.5 V, are shown in Fig. 5(a) and Fig. 5(b), respectively. The optical spectrum exhibits an optical frequency comb as the optical modes are evenly spaced, with the similar suppression ratio ∼30 dB. By contrast, the comb becomes much wider and having more comb lines than that in Fig. 4(a). The emission wavelength also shifts to about 1555 nm, that is ∼20 nm away from the emission wavelength of the previous comb. Since there are 25 comb lines within the interval of 1550 – 1570 nm, the fitting curve based on the same algorithm becomes more reliable. With the fitting curve, the estimated 3-dB spectral bandwidth is calculated to be ∼0.7 THz. The trace in Fig. 5(b) shows an isolated spike indicative of a pulse. In accordance with the wider comb in Fig. 5(a), the trace becomes much narrower than that in Fig. 4(b). The zero background indicates a pedestal-free complete mode locking. And the FWHM of trace is 0.71 ps from the fitting Gaussian curve. With a deconvolution factor for Gaussian function 0.707, the retrieved FWHM pulse width is 0.50 ps. It is three times narrower than the pulse width in Fig. 4(b). As shown previously, when properly biased 100-GHz-spacing combs and 500-femtosecond pulses are obtained. Please notice that there is a ∼20 nm wavelength jump between the two combs at the selected bias conditions, which is explained in next section.
Fig. 5 (a) Optical spectrum with ISOA = 99 mA and VSA = 2.5 V. (b) Autocorrelation trace ISOA = 99 mA and VSA = 2.5 V.
Figure 6 shows the evolution of spectrum in dependence of ISOA. A series of optical spectra is presented with ISOA varied from 30 to 150 mA and VSA fixed at 1 V. When ISOA = 30 mA, no lasing is observed, as the device is operated below threshold. When ISOA = 40 mA, the device starts lasing around 1567 nm, as the device is just above threshold. As the forward bias current ISOA increases, the multimode emission spectrum shifts to the higher wavelength around 1572 nm. This red shift results from the carrier heating in gain sections. When ISOA is within 80 mA – 100 mA, a few optical modes evenly spaced by 0.8 nm are clearly observed, similar to the optical spectrum shown in Fig. 4(a).
When ISOA increases from 100 mA to 110 mA, the 0.8-nm (100-GHz) comb around 1572 nm collapses and one dominant lasing mode grows around 1578 nm. Furthermore, when ISOA is within 120 mA – 140 mA, broad spectra without harmonic selection are observed. The mode spacing is 25 GHz, corresponding to the fundamental round trip frequency of the 1.66-mm Fabry-Pérot cavity, indicating that the device is in the continuous-wave (CW) Fabry-Pérot operation. Finally, when ISOA = 150 mA, the mode suppression behavior is absent and one can observe the outline of the emission spectrum, indicating the device is in the self-pulsation/Q-switching operation. The mutual coherence information among the 100-GHz optical comb lines can be extracted from mode beating radio-frequency (RF) spectrum, which can be carried out with a high-speed > 100 GHz photodiode and an ultrawide-bandwidth > 100 GHz electrical spectrum analyzer. The different operation regions at different bias conditions are related to the change in slope efficiency of L-I curve as shown in Fig. 3(a).
Similarly, the evolution of spectra as the reverse bias voltage VSA increases is shown in Fig. 7. In this sequence of optical spectra, the forward bias current ISOA is fixed at 100 mA above threshold, and the reverse bias voltage VSA increases from 0 to 6 V in a step of 0.5 V. When VSA = 0 V, the spectrum in the multimode Fabry-Pérot operation is observed, with 25-GHz mode spacing around the wavelength of 1573 nm. When VSA = 0.5 V and 1 V, optical spectra containing few modes evenly separated by 100 GHz are presented, similar to those shown in Fig. 7 with ISOA is around 100 mA. When VSA increases from 1 V to 1.5 V, the optical spectrum exhibits multiple peaks in a distorted outline. There is a ∼20-nm wavelength jump between the spectra at VSA = 1.5 V and VSA = 2 V.
This is because the absorbers are red-drifted. The SA bandgap goes to the red with quantum-confined stark effect-induced shift due to the increased bias voltage. And the spectrum jumps from the regime with minimal losses around the SA band-edge towards the peak of gain in the shorter wavelength, which is around 1555 nm – 1560 nm in this case. Hence, the ~20-nm wavelength jump between VSA = 1 V and VSA = 2.5 V is observed. Apart from the wavelength jump, the transition of regimes occurs from 1 V to 1.5 V and from 2 V to 2.5 V. In the region within 1 – 2 V, the shape is indicative of self-pulsation/Q-switching where the pulse trains are modulated at the relaxation oscillation frequency, usually a few GHz [29].
Above VSA = 2.5 V, wide combs around 1560 nm are achieved, similar to that has been shown in Fig. 5(a). When VSA > 3.5 V, there are few random lasing modes existing around the same wavelength range. While VSA = 5 V, only two modes occur, with a separation of 1.3 THz. More exemplary dual-mode spectra are experimentally demonstrated along with autocorrelation traces in next section. While VSA > 5 V, it seems two groups of modes at 1555 nm and 1565 nm, for which further investigation is required.
The device is capable of generating mode-locked laser. And it can also generate dual-mode laser signals at various rates as shown in Fig. 8 and Fig. 9. When the laser is operated with higher bias levels, it generates higher-order harmonics [8]. In the THz region, the spectral separation between optical lines is too large (> 8 nm) so that the number of optical lines becomes smaller. In such conditions, only two optical modes instead of a huge number of evenly spaced comb lines take place. In Fig. 8(a), two optical lines separated by 1 THz with ~30-dB suppression ratio are shown. And Fig. 8(b) shows the corresponding autocorrelation trace in the period of 1 ps. As there are only two optical lines on the wavelength domain as shown in Fig. 8(a), no matter if they are phase-locked, the autocorrelation trace gives a sinusoidal wave rather than a pulse train. This 1-THz dual-mode laser is achieved with ISOA = 86 mA and VSA = 3.9 V. The bias condition for the 1.2-THz dual-mode signals shown in Fig. 8(c) is ISOA = 99 mA and VSA = 4.4 V. Figure 8(d) shows the sinusoidal trace in 0.83-ps period. Fig. 8(e) is ISOA = 140 mA and VSA = 4.1 V. Figure 8(f) shows the sinusoidal trace in 0.77-ps period. Please notice that for these three THz-rates the SA sections are biased with relatively high voltages (>4 V).
Fig. 8 (a) Optical spectrum, and (b) autocorrelation trace for 1 THz dual-mode emission. (c) Optical spectrum, and (d) autocorrelation trace for 1.2 THz dual-mode emission. (e) Optical spectrum, and (f) autocorrelation trace for 1.3 THz dual-mode emission.
Fig. 9 (a) Optical spectrum, and (b) autocorrelation trace for 2 THz dual-mode emission. (c) Optical spectrum, and (d) autocorrelation trace for 2.2 THz dual-mode emission. (e) Optical spectrum, and (f) autocorrelation trace for 2.7 THz dual-mode emission.
Similarly, Fig. 9(a), Fig. 9(c), and Fig. 9(e) show other three different dual-mode laser signals at even higher THz-rates at 2 THz, 2.2 THz, and 2.7 THz. And their time-domain signals are shown in terms of autcorrelation traces such as Fig. 9(b) in 0.5-ps period), Fig. 9(d) in 0.45-ps period, and Fig. 9(f) in 0.37 ps. All the THz-rates in the exemplary spectra are integer multiple of the fourth harmonic 100-GHz according to the cavity configuration. These higher-order harmonic signals can be used for THz CW generation by photomixing, with the two modes being beat with a photodiode or photoconductor [24].
4. Conclusion
We have successfully presented an InP mode-locked semiconductor laser for 100-GHz optical frequency comb generation. The pulsewidth amounts into the femtosecond scale - 0.5 ps. Dual-mode generation is also experimentally demonstrated at the various THz rates up to 2.7 THz with higher bias levels. The laser cavity features multiple segments in a symmetric arrangement in which short SOAs separate the overall 1.66-mm-long resonator into four shorter divisions in equal length to achieve fourth harmonic generation. With the generic foundry approach, we have made use of predefined building blocks such as passive waveguides, multimode interference reflectors as on-chip reflectors, and gain/absorber sections (SOA) to realize the design geometry that is fully compatible to the foundry’s standard fabrication process in multi-project runs. The on-chip multimode interference reflectors have shown the potential to replace the commonly used but not reliable cleaved facets.
Funding
European Union’s Horizon 2020 (Marie Skłodowska-Curie grant agreement No. 642355 Fi-WiN5G), Spanish Ministerio de Economia y Competitividad thourgh Programa Estatal de Investigación, Desarrollo e Inovación Orientada a los Retos de la Sociedad thougth grant iTWIT, TEC2016-76997-C3-3-R.
Acknowledgments
The authors thank Dr. Julien Javaloyes for the helpful discussions. This work has been supported by European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 642355 FiWiN5G.
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