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Abstract
We report a novel single-cavity dual-wavelength laser that has two distributed Bragg reflector (DBR) gratings at each side of a gain section for THz communication applications. By varying the inject current of one of the DBR gratings, the optical beat frequency of the laser can be widely tuned. In the device, a high-speed electro-absorption modulator (EAM) is also integrated and can be used for up to 25 Gb/s data modulation.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In recent years, the use of millimeter wave (MMW) and terahertz (THz) wave frequency range (from 30 GHz to 1 THz and above) has been widely studied to meet the rapidly increasing data rate of future wireless communications [1]. The carrier frequency basically determines the data rate especially in simple modulation schemes, which have the advantage of low latency. For the generation of continuous wave (CW) high frequency carriers, photonic technologies [2] have several advantages including easy access of high quality high frequency components, low loss distribution of high frequency carrier signals over fiber and high Electro-Magnetic Interference (EMI) immunity, when compared with electronic techniques such as Gunn diodes, avalanche transit time diodes, and frequency multipliers [3].
A typical photonic technique for generating CW MMW or THz wave is optical heterodyning, which is simple to be implemented and offers wide frequency tuning range. In the technique, two laser beams mix in a photomixer such as a high speed photodiode to generate an electrical signal whose frequency equals the wavelength difference. The interference of two laser beams leads to the intensity modulation of the laser beams with a THz frequency equal to the frequency difference between the two lasers. When the modulated laser beams are fed into a photodiode, electrons and holes are generated THz-periodically. Collective currents of these carriers are fed into an antenna, form which CW THz emissions are radiated. In [4], two commercial thermally tunable distributed feedback (DFB) lasers are used to generate THz carrier with frequency at 138 GHz for transmitting uncompressed 4 K video signal. In [5], two narrow linewidth lasers with extended cavity are used to setup a THz wireless transmission system at 400 GHz carrier frequency, which supports up to 40 Gb/s date rate. Though the application of discrete lasers have been successfully demonstrated, monolithically integrated dual wavelength laser sources fabricated by photonic integration techniques [6] have been attracting great interests recently. By integrating different optical components into a single chip, advantages such as cost effective production, small system size, low power consumption, and lower optical coupling loss can be achieved. Besides, on a single chip, two laser wavelengths experience the same fluctuation of temperature, helping to lower the carrier phase noise.
Up to now, different types of integrated dual wavelength laser source have been presented. Kim et al. reported a dual wavelength laser [7], which has a DFB laser integrated with a distributed Bragg reflector (DBR) and can produce widely tunable optical beat frequency. Carpintero et al. integrated two side by side tunable InP/polymer DBR lasers into a hybrid photonic integrated circuit [8], forming a dual wavelength source, which can be stabilized through optical injection. Integrated light sources including two side by side thermally tunable DFB lasers and a passive optical combiner have also been reported [6,9–11]. Other types of integrated dual wavelength sources include single [12–14] or two section [15] dual wavelength DFB semiconductor lasers, heterogeneous quantum dot laser diode with a silicon external cavity [16], multi-channel DFB laser arrays [17,18]. Besides semiconductor lasers, dual wavelength DFB fiber lasers have also been fabricated [19,20].
In this paper, we report a novel dual wavelength laser for THz communication applications, which has several advantages. First, in the device, a high performance electro-absorption modulator (EAM) having 25 Gb/s modulation speed is integrated monolithically to modulate the dual wavelength emission for the first time, to the best of our knowledge. In contrast, in most of the earlier studies, the transmitted data is intensity modulated onto the THz carriers through a separate commercial modulator [4,5,8,21], which has disadvantages such as increases of system complexity and power consumption. In the device reported in [6], an integrated semiconductor optical amplifier (SOA) is used for modulating the two wavelengths at the same time. However, SOA is not a good modulator and the reported data rated is only 1 Gb/s. Then, the optical beat frequency of the laser can be widely tuned by varying the inject current of DBR gratings. Compared with most previously reported monolithic dual wavelength semiconductor lasers, in which the beat frequency is tuned by thermal effects from either integrated thin film heaters [14,15] or self-heating effects induced by gain current [10,17,18,21], wavelength tuning by changing the DBR current has less effects on laser properties such as threshold current and light output power, because the gain bias condition needs not to be changed and self-heating of the DBR section has only small effects on the gain section [22]. Besides, possible optical power variation can be compensated effectively by the integrated SOA of our device. Finally, when compared with other types of integrated dual wavelength sources such as side by side two lasers [6] or multi-channel laser arrays [17], our device has a simple single cavity structure, which helps to reduce the chip cost.
2. Device structure and fabrication
Figure 1(a) shows an optical graph of a fabricated device, which consists a four section DBR laser and a 150 µm EAM. A 500 µm SOA is integrated before the EAM to boost the optical power. The DBR laser includes a 200 µm rear DBR (RDBR) section, a 100µm phase section, a 400µm gain section and a 100 µm front DBR (FDBR) section, respectively. The device is fabricated with InP based materials, which are grown by a three step metal organic chemical vapor deposition (MOCVD) process. The first MOCVD step is a selective area growth (SAG) process, in which a 50 nm detune of peak photoluminescence (PL) wavelength between the multi-quantum well (MQW) material for the EAM and the MQWs for the the laser or semiconductor optical amplifier (SOA) is obtained. The peak PL wavelength of the EAM MQWs is set as 1.49 µm. Then, the MQWs in the DBR and phase sections are removed selectively. In the following MOCVD step, bulk InGaAsP layers with 1.45 µm band-gap wavelength (1.45 Q) is butt-jointed as the DBR or phase material. The large refractive index of the long wavelength grating material helps to enlarge the wavelength tuning range [23]. Gratings are formed in the InGaAsP layer of the two DBR sections by combining holographic expose and dry etching. A p type InP cladding layer and InGaAs contact layer are grown successively in the last growth step. The schematic coss-section material structure is shown in Fig. 1(b). Detailed fabrication process of the device can be found elsewhere [23–25]. The butt-joint process has been optimized carefully. By proper design of the butt-joint mask and optimizing the etching conditions, the residual reflections from the butt joint facet have been well suppressed so that no effects on the tuning properties of the laser can be observed [24,25].
Fig. 1. (a) Optical graph of a fabricated device, (b) The schematic cross-section material structure.
As shown in Fig. 1(a), the waveguide of the SOA is curved to obtain a 7 degree angle of the EAM facet in order to lower facet reflections. The EAM and rear DBR facets are coated with AR and HR coatings, respectively. In the laser shown in Fig. 1(a), the front and rear DBR sections have separate electrodes, whose inject current can be controlled independently. Besides, the length of the front DBR section is 100 µm, which is longer than the device in [23] and is long enough to support single wavelength emission. As a result which is shown below, the laser emits dual wavelengths, which are attributed to the front and rear DBR gratings, respectively. In the DBR lasers reported in [23], however, the electrodes of the front and rear DBR sections are connected and the DBR laser have single wavelength emission, which is typical for conventional optical communication applications. In the device reported here, the thickness of the polymer layer under the EAM electrode is made larger than the previous device [23] to enlarge the modulation bandwidth.
3. Device characterizations
The laser chip is mounted on a high frequency heat sink for characterizations. A thermoelectric cooler (TEC) is used to keep the temperature of the chip at 25 oC during the test. Figure 2 shows the light output power as a function of the gain current measured by an integrating sphere. During the test, there is no current in the DBR sections and the phase section and the EAM is not biased. The inject current in the SOA section is fixed at 150 mA. As can be seen, the threshold current is about 17 mA. At 150 mA gain current, the output light power is about 2.9 mW, which is notably smaller than the single wavelength DBR device in [23], which has a 50 µm long front DBR section. In the present device, for the front DBR grating to support a single wavelength emission alone, its length is increased to 100 µm. The resulting larger reflectivity reduces the light power that is injected into the SOA.
Fig. 2. Light output power as a function of the gain current. During the test, there is no current in the DBR sections and the phase section and the EAM is not biased. The inject current in the SOA section is fixed at 150 mA.
In the device, the two reflectors on the two sides of the gain section form a Fabry-Perot (FP) cavity which supports a number of FP modes having different wavelengths. The Bragg wavelength of the DBR reflector λB = 2 neff ×Λ, in which neff is effective refractive index of grating material and Λ is grating pitch. When current is injected only into one DBR section, neff of the DBR material and thus λB are reduced because of the free carrier plasma effect [23]. As a result, each of the two reflectors selects a different laser wavelength from the set of FP modes, resulting in two mode emission at the same time. Besides selecting its own laser wavelength, each DBR section also provides reflection to support the other wavelength mode.
Through SEM measurements, the depth and duty ratio of our DBR grating are determined to be 30 nm and 45%, respectively, which resulting in a 120 cm-1 coupling coefficient. Using the value, the reflection spectra of the front and back DBR gratings are calculated [26] and shown in Fig. 3(a). For the front grating the Bragg wavelength is chosen to be 1558 nm. For the rear grating, the Bragg wavelength is chosen to be 1553 nm, to show a state when a current (about 11 mA) is injected into the rear DBR section. The peak reflectivities of the rear and front gratings are 97% and 70%, respectively. As shown schematically in Fig. 3(a), the short wavelength mode λ1 is selected by RHλ1 provided by the rear grating and is supported by RLλ1 provided by the front grating. In a similar manner, the long wavelength mode λ2 is selected by RHλ2 provided by the front grating and is supported by RLλ2 provided by the rear grating. When the separation between the two modes is larger than a 3.8 nm, which is the difference between the peak reflection wavelength and the first reflection minimum wavelength of the front DBR grating, both RLλ1 and RLλ2 are supplied by the lobes of the spectra as the case shown in Fig. 3(a). Except in the narrow wavelength ranges between each two lobes where the reflectivities are approaching 0, the reflectivities are large enough to support DBR laser modes in up to 10 nm range on each side of the spectra. For lasers having high reflection (such as > 70% in our device) on one facet, only a weak reflection is needed for the other facet to obtain laser operation. In a reference work it is shown that a low threshold current can still be obtained for a DBR laser having 1% reflection on one facet [27]. In Fig. 3(a), the wavelength range in which the reflectivities are smaller than 1% is less than 12 percent of the whole calculated 20 nm range.
Fig. 3. (a) Calculated reflection spectra of the gratings of our device. (b) Spectra behavior when the front grating current is 0 mA and the rear grating current is varied from 0 to 80 mA. During the measurements, the currents in the gain and SOA sections are 100 and 110 mA, respectively. The EAM is also not biased. The dashed blue curve is the reflection spectrum of the rear DBR grating when there is no current injected.
When no currents are injected into the two separate DBR sections, the two DBR gratings have the same λB. Thus the laser has a single wavelength emission. In the DBR laser we reported in [23], because the length of the front DBR reflector is smaller than that of the laser in this work, though the FP modes are also there, the weaker reflection does not allow the reflector to support its own emission wavelength, leading to the single mode operation of the laser.
The spectra behavior when the front grating current is 0 mA and the rear grating current is varied from 0 to 80 mA is shown in Fig. 3(b). During the measurements, the currents in the gain and SOA sections are 100 and 110 mA, respectively. The EAM is also not biased. As can be seen, the wavelength of the mode selected by the rear grating decreases gradually with the current which is similar to a normal DBR laser [23]. As a result, the wavelength separation between the two modes increases from about 0.9 nm at 3 mA grating current to about 10.2 nm at 50 mA grating current. Further increase of the current leads to no enlargement of the separation because the blueshift of the wavelength becomes saturated. The frequency tuning range of our device is larger than those of the devices reported in Refs. [6,9–11], but is smaller than the range of the DFB laser integrated with a DBR grating reported in Ref. [7]. Because of the large refractive index of the DBR grating material, over 12 nm wavelength tuning range have been obtained in the single wavelength DBR laser as presented in Ref. [23]. Both the smaller wavelength tuning range and the irregular mode jumping during the tuning shown in Fig. 3(b) can be attributed to the large width of the reflection spectra, which leads to poor mode selection [26]. The performance can be improved by a sharp reflection peak which can be obtained by using gratings having low coupling coefficient and long length [26]. As there is current injected into the rear DBR section, the free carrier absorption effects will decrease the light intensity in the device cavity. Thus the light intensity in the front DBR section is also affected, which leads to the mode jump of the long wavelength emission as the rear DBR current is varied shown in Fig. 3(a). It is worth noting that there might be residual reflections from the butt joint facet, which may result in the irregular wavelength tuning of the short wavelength mode as shown in Fig. 3(b).
In Fig. 4 detailed properties of three typical dual wavelength modes that can be obtained from the device are shown. As shown in Fig. 4(a), when the rear grating currents are set as 3.8, 11.3 and 31 mA, the wavelength separations are 0.93, 4.26 and 8.64 nm, respectively. Figure 4(b) shows the autocorrelation traces corresponding to the different working modes in Fig. 4(a). The THz repetition frequencies measured from Fig. 4(b) are 0.12, 0.53 and 1.08 THz, respectively, for the DBR currents of 3.8, 11.3 and 31 mA, which agree well with the wavelength differences shown in Fig. 4(a).
Fig. 4. (a) Dual wave length emission properties of the laser at different rear DBR current (mA), the wavelength separation between the two modes is marked in nanometer, (b) the corresponding autocorrelation traces, (c) THz power as a function of frequency. During the measurements, the currents in the gain, SOA and front DBR sections are 100,110 and 0 mA, respectively. The EAM is not biased.
CW THz generation using the DBR laser is conducted by using a commercial antenna integrated uni-Traveling carrier photodiode (UTC-PD) (NTT IOD-PMAN-13001) as a mixer. Before being coupled into the UTC-PD, the light from the DBR laser is amplified first by an erbium-doped fiber amplifier (EDFA) and then modulated by a optical chopper at 15 Hz. The THz power from the UTC-PD is focused into a Golay-cell with THz lenses. During the measurements, the photocurrent of the PD is kept at 3.0 mA. Standard single mode fiber is used in the test system. The measured THz power as a function of frequency is shown in Fig. 4(c). As can be seen, the THz power decreases from 10.0 µW at 0.12 THz to 1.8 µW at 1.08 THz, which can be attributed to the limited bandwidth of the UTC-PD. Limited by the speed of carrier transport and RC constant, the radio frequency response of the PD decreases with the beating frequency gradually. Similar results have also been observed in Ref. [7].
Figure 5 shows the effects of the SOA current on the properties of the dual mode emission. As is shown, as the SOA current is 85 mA, the intensity of the short wavelength mode is about 20 dB higher than the long wavelength mode. As the SOA current is increased to 100 mA, the intensity difference is decreased to be less than 1 dB. When the SOA current is further increased, the intensity of the long wavelength mode starts to be higher than the short wavelength mode, leading to an increasing intensity difference. These results show that, besides optical power booming, the SOA also helps to obtain a balance optical power between the two beating modes, which is key to get a high conversion efficiency between optical power and THz power. Because of gain saturation of the SOA, the gain for the mode with higher intensity is smaller than the gain for the mode with low intensity. As a result the intensity difference after the SOA can be reduced.
Fig. 5. The effects of SOA current on the dual wavelength emission. During the measurements, the currents in the gain and rear and front DBR sections are 100,28 and 0 mA, respectively. The EAM is not biased.
By optically down converting a 0.135 THz (1.08 nm dual wavelength separation) signal which is obtained by setting the rear and front DBR current at 4.0 and 0 mA, respectively, to 38.8 GHz, the line width of the THz signal is measured by using an electrical spectrum analyzer and a PD as a mixer [20]. A commercial intensity modulator (EOspace) is used to modulate the light to generate side bands. The electrical spectra of the down converted signal are shown in Fig. 6. The measured 3 dB linewidth is about 34.5 MHz. To get THz carriers with low noise, optical injection technique can be used [8].
Fig. 6. Electrical spectra of a 0.135 THz signal down converted to 38.8 GHz optically. (a) Spectrum measured with a 20 GHz span, (b) detailed spectrum measured with a 300 MHz span, the blue curve is the Lorenz fit of the measured data. During the measurements, the currents in the gain and SOA sections are 100 and 110 mA, respectively. The currents in the rear and front DBR sections are 4.0 and 0 mA, respectively. The EAM is not biased.
As shown in Fig. 4(a), there are side modes shown in the optical spectra of the device, whose intensities are 23 dB (the top spectrum) and 27 dB (the bottom two spectra) smaller than the intensities of the corresponding dual mode emissions. To reduce the effects from these modes, higher side mode suppression ratios (SMSR) can be obtained by reducing the length of the gain and phase sections, that is, the length of the FP cavity formed by the rear and front DBR reflectors. By doing so, the separation between each two FP modes can be increased, thus the side modes can be better suppressed. The SMSR can also be improved by using lower coupling coefficient and longer grating length to obtain a narrow reflectivity bandwidth [26], which favors better mode selectivity. The wavelength tuning range and thus the THz frequency tuning range can be enlarged at the same time because of the narrow reflectivity bandwidth. Under the premise of enough mode selectivity, the length or coupling coefficient of the front DBR grating can be reduced so that the light output power can be increased. The separations between the laser modes and the major side modes are larger than about 0.4 nm (50 GHz). The effects of the side modes can be also suppressed by using both optical filters and radio frequency filters.
In most of earlier studies, a separate commercial modulator is used to modulate the transmitted data onto the THz carriers [4,5,8,21], which leads to high system complexity and power consumption. In the present device, we monolithically integrate a high performance EAM for data modulation to enhance the system performance. The static optical extinction properties of the EAM are shown in Fig. 7(a). Under different dual mode conditions, larger than 20 dB extinction ratio (ER) can be obtained at -5 V bias voltage. The small signal modulation responses of the EAM under -1, -2 and -2.5 V reverse bias voltages are shown in Fig. 7(b). As can be seen, the modulation responses are relatively flat at up to 22 GHz for all the applied voltages. The modulation bandwidth can be further increased by reducing the length of the EAM and the increasing thickness of the polymer layer under the modulator contact pad to reduce the capacitance.
Fig. 7. (a) Static optical ER of the EAM when different currents are injected into the rear DBR sections, (b) modulation response of the EAM when it is bias at different voltage and the rear DBR current is 11.3 mA. During the measurements, the currents in the gain, SOA and front DBR sections are 100, 110 and 0 mA, respectively.
10 Gb/s and 25 Gb/s data modulation experiments are conducted in a back to back (B2B) configuration. During the measurements, the EAM is biased at -3 V and the separation between the dual modes is set at about 4 nm (about 300 GHz). The modulation voltages (Vpp) for 10 and 25 Gb/s modulation are 1.6 and 2.4 V, respectively. Figure 8 shows the B2B eye diagrams measured by a Keysight DCA-X 86100D oscilloscope. As can be seen, the eyes are clearly opened for both 10 Gb/s and 25 Gb/s data modulation. What is more, the dual mode emission is stable during the modulation tests. For our device, the integration of EAM eliminates the needs of a separate modulator, which will reduce the system complexity and reduce the optical power loss related to the package of discrete optical elements.
Fig. 8. Eye diagrams obtained at 10 Gb/s, Vpp = 1.6 V (a) and 25 Gb/s Vpp = 2.4 V (b) modulation in the B2B condition. During the measurements, the currents in the gain and SOA sections are 100 and 110 mA, respectively. The currents in the rear and front DBR sections are 11.3 and 0 mA, respectively. The EAM is biased at -3 V. The rms jitters are 1.97 and 1.82 ps for the 10 and 25 Gb/s eye diagrams, respectively.
4. Conclusion
A novel single cavity dual wavelength laser which has two DBR gratings at each side of a gain section is fabricated. By varying the inject current of one of the DBR gratings, the optical beat frequency of the laser can be widely tuned. In the device, a high speed EAM is also integrated and can be used for up to 25 Gb/s data modulation. The integrated dual wavelength laser is promising for THz communication applications.
Funding
National Natural Science Foundation of China (61635010, 61320106013, 61474112, 61574137); National Key Research and Development Program of China (2018YFB2200801,2016YFB0402301).
Disclosures
The authors declare no conflicts of interest.
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