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A monolithically integrated low linewidth optical comb is demonstrated by gain switching of a three-section laser device. The device consists of a slave and master section separated by a shared slotted mirror section. Wavelength tunability has been demonstrated by varying the electrical bias of each section. The number of comb lines is shown to almost double with the addition of optical injection from the master section into the slave. The unmodulated device has a full width half max linewidth of ∼ 500 kHz, while the comb line set were measured to be ∼ 600 kHz, with little degradation as a result of gain switching. The FSR (free spectral range) of the demonstrated comb is 4 GHz, which is tunable within the bandwidth of the device, with a central wavelength of 1580.3 nm.
© 2016 Optical Society of America
1. Introduction
Optical comb sources have applications in spectroscopy [1], space-based instruments [2], and high speed telecommunications [3, 4]. An optical comb featuring a low linewidth is necessary for advanced modulation formats in Tbit/s coherent superchannel communications systems [5, 6]. A monolithically integrated phase and modulation section using quantum well intermixing has been previously demonstrated which showed a linewidth for each comb line of 25 MHz limited by the laser itself [7], although this linewidth is much too wide to be used to generate coherent superchannels. In comparison, directly modulated (gain switched) lasers are of interest for generating narrow linewidth combs due to their tunable wavelength, free spectral range (FSR), and ease of integration [8, 10]. In contrast to this; comb generation based on mode-locked lasers that have a fixed FSR [11], and electro optic modulators which are difficult to integrate with lasers [12].
Injection locking from an external master laser has been shown to reduce phase noise [9], and thus linewidth in gain switched lasers. In this paper we demonstrate the integration of a master laser without an isolator with the gain switched laser in order to reduce output linewidth. Two Slotted Fabry-Pérot (SFP) lasers [13] are monolithically integrated in a strongly coupled master/slave configuration, whereby the slave laser is optically phase-locked to the master laser. On-chip optical phase-locking has been demonstrated to improve laser characteristics, such as increased side mode suppression ratio (SMSR), and enhanced relaxation oscillations [14, 15]. They are operated in an asymmetric bias regime, where the master is more heavily biased than the slave. The slave laser is then gain switched using a high power RF signal generator. The output of the slave laser is analyzed using a Yokogawa AQ6370D optical spectrum analyzer (OSA), which has a resolution of 0.02 nm, in order to observe the number of comb lines and their spacing.
2. Device design
Single facet SFP lasers rely on a series of periodically etched slots to provide optical feedback in place of one of the cleaved facets. These slots provide electrical isolation between the sections of the device, thus allowing independent biasing [16]. The device described here features two single facet SFP lasers integrated in such a way that they share a slotted mirror section. By sharing a common mirror section, the total length of the device is reduced to ∼2 mm. The mirror section has a length of 756 μm, with the master/slave sections 800 μm and 400 μm respectively. The inter-slot distance is 108 μm.
The device was fabricated on commercially available lasing material consisting of 5 compressively strained AlGaInAs quantum wells on an n-doped (100) InP substrate, with a total active region thickness of 0.41 μm. The upper p-doped cladding consists of a 0.2 μm InGaAs cap layer, followed by 0.05 μm of InGaAsP, lattice matched to 1.62 μm of InP. Standard lithographic techniques were used to define the ridge and slot features, with a ridge width of 2.5 μm, height 1.7 μm, and a slot width of 0.88 μm, with the ridge etch stopping above the quantum wells. A separate etch through the quantum wells was performed in order to provide access to the n region. Benzocyclobutene (BCB) was used to planarize the structure before metal contact deposition. BCB has a low dielectric constant which reduces the capacitance between the contact pads and the n-doped substrate by increasing the distance between them. A lower capacitance reduces the RC limit on the modulation of the device [17], ensuring higher modulation speed can be achieved. Ground-signal (GS) contacts were deposited in order to allow high-speed modulation of the device. The ground pad makes contact with the n-type substrate through an etched via. The structure of the device can be seen in Fig. 1.
Fig. 1 (a) Top profile of device. GS contact pads can be seen for both the master and the slave. (b) Side profile of device. Slotted mirror section can be clearly seen. Slot depth is equal to the ridge height.
3. Device testing
3.1. DC characterisation
The DC characteristics of the device were first examined. The device was mounted on a temperature controlled chuck, maintained at 24 °C, and lensed optical fiber was used to couple light from the device.
Slave and mirror sections were biased at 20 mA and 30 mA respectively, with the master section unbiased. An optical spectrum was recorded using an OSA. The master section was then switched on with a bias of 50 mA. The optical spectrum of the slave output is shown in Fig. 2(a) for master on/off. Single-mode behavior was observed with a high SMSR (> 30 dB) with the slave optically phase locked to the master. By varying the bias of the master and mirror sections, 4 main lasing modes were obtained. The tunability of the device can be seen in Fig. 2(b), with lasing modes at 1573.1 nm (slave: 20 mA, mirror: 17 mA, master: 40 mA), 1576.5 nm (slave: 20 mA, mirror: 30 mA, master: 30 mA), 1579.7 nm (slave: 20 mA, mirror: 30 mA, master: 50 mA), 1583.9 nm (slave: 20 mA, mirror: 14 mA, master: 51 mA). The tuning jumps of roughly 3.2 nm (400 GHz channel spacing) are defined by the interslot distance in the mirror section stated in the previous section. The mode intensities could be balanced with the addition of a semiconductor optical amplifier (SOA) integrated with the laser in future designs [13].
Fig. 2 (a) Slave output before/after master section injection. (b) Optical spectrum on varying master and mirror bias showing 4 main lasing modes.
3.2. Modulation bandwidth and comb generation
The modulation bandwidth of the device was investigated by measuring the S21 (transmission) response of the slave section. The S21 is a measure of how much RF electrical power is transmitted through a device, and is useful for deducing the maximum RF frequency supported by a device. The S21 response was obtained using an Agilent 8753ES two-port vector network analyzer (VNA). The VNA sent a low power RF sinusoidal signal of varying frequency through the device from port one, which modulates the optical output of the device. The optical signal was converted to an electrical signal using a photodiode (PD) and recorded through port two of the VNA. The VNA had a range of 30 kHz to 6 GHz. A GS probe was used to make contact with the slave section and a DC bias was simultaneously applied via a bias tee.
The S21 response of the device can be seen in Fig. 3(a). For a slave and mirror bias of 40 mA and 50 mA, the resonance frequency of the slave was measured to be 2.5 GHz, with a 3 dB drop off at 3.5 GHz. For RF signals above 3.5 GHz, the signal experiences an increasing loss in the device. The resonance frequency was increased to 3.5 GHz, with a 3 dB drop off at almost 5 GHz by biasing the slave section at 20 mA, the mirror section at 30 mA, and by biasing the master section at 50 mA.
Fig. 3 (a) Resonance frequency enhancement. (b) Optical comb with master ON/OFF, with an FSR of 4 Ghz. The master ON comb is centered at 1580.3 nm, the master OFF comb at 1586.9 nm. This difference arises as the slave section required higher biasing to generate a comb with the master OFF.
To generate an optical comb, the slave section was gain switched by applying a high power RF sinusoidal signal (> 20 dBm) at 4 GHz from a signal generator in order to directly modulate the current. Due to the impedance mismatch between the 50 Ohm signal generator and the laser as evaluated from the S11 VNA measurements, we estimate that 40% of the RF power was absorbed by the laser. A 4 GHz modulation frequency was chosen as this value is within the bandwidth of the device when all three sections are biased, but exceeds the bandwidth with the master section unbiased. The slave laser DC bias was tuned in order to obtain the best comb (slave: 40 mA, mirror: 54 mA), with the master section unbiased. The output of the slave was recorded using an OSA. The master section was switched on and all three sections tuned in order to obtain the best comb (slave: 20 mA, mirror: 54 mA, master: 30 mA). The optical combs obtained from both of these configurations can be seen in Fig. 3(b). An almost 2x increase in the number of comb lines is obtained with the master section switched on. We observe 8 comb lines within a 3.5 dB band of each other, as opposed to 3 lines for the slave and mirror comb. The comb generated with all three sections biased was observable from both device outputs, with no apparent asymmetry.
3.3. Linewidth measurement
The linewidth of the device was measured using the recirculating delayed self-heterodyne interferometer (RDSHI) method [18]. A schematic of the measurement setup can be seen in Fig. 4(a). Light was coupled from the laser into a 90/10 splitter whereby 90% of the power went through the 50 km of fiber loop, and 10% is passed to a photodiode (PD). In the loop, the laser was frequency-shifted 80 MHz with an acoustic optic modulator (AOM), and passed through a polarization controller. The frequency shifted beam then recombines with the unshifted beam at the PD and the beat note at 80 MHz between the beams can be analyzed on an electronic spectrum analyzer (ESA). Higher order beat notes with decreasing power can be observed due to recirculation in the loop. As a delay length of 50 km gives a resolution of < 2 kHz, which was more than sufficient for this device, only the beat note at 80 MHz was recorded.
Fig. 4 (a) Schematic for the RDSHI linewidth measurement. (b) Measured linewidths for the device (modulated/unmodulated), and commercial DFB. 3 dB width of the comb set linewidth can seen in the top left inset.
The beat note for the unmodulated device, the gain switched device, and a commercial DFB (JDSU CQF915) laser can be seen in Fig. 4(b). The full width at −3 dB from the peak was measured as: ∼ 500 kHz for the unmodulated device (slave: 20 mA, mirror: 30 mA, master: 50 mA), ∼ 600 kHz for the gain switched device (slave: 20 mA, mirror: 54 mA, master: 30 mA), and ∼ 2.6 MHz for the DFB. The linewidth of the comb set was found not to broaden significantly as a result of gain switching, with the measured linewidth considerably lower than that of the DFB laser. The comb set full width linewidth of 600 kHz is ideal for current state of the art 16-QAM (quadrature amplitude modulation) coherent communication systems which have a linewidth tolerance of 1.4 MHz [19].
4. Conclusion
A low linewidth optical frequency comb has been demonstrated using the gain switching technique, via the integration of SFP lasers into a three section device. This device has been shown to have a highly single mode output, which is ideal for gain switching. The master slave injection results in an increased modulation bandwidth compared to a standard 2 section SFP laser. The comb set generated from this device have been shown to be low linewidth (∼ 600 kHz), making this device an ideal candidate for next generation transmitters which make use of advanced modulation formats. This work was supported by the Science Foundation Ireland under grants 12/RC/2276 (IPIC), and SFI10/CE/I1853 (CTVR).
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