1. Introduction

In recent years, optical generation of millimeter-wave (MMW) signals (30 GHz – 300 GHz) have been attracting considerable interest from both academia and industry due to its advantage of simple, flexible and scalable system design for achieving very high frequencies with very low noise. In particular, applications in future seamless fiber-wireless integrated access networks, such as fronthaul for 5G and beyond mobile wireless networks [1–4], are of interest. However, MMW signals in the optical domain are commonly generated by using commercially available single frequency laser sources, which might not achieve MMW signals with high spectral purity due to the lack of coherence and correlation between the laser sources themselves. Relative drift in emission wavelength between the sources will cause broadening/degeneration of the MMW signal, hence, degrading the system performance. It has been recently shown that the performance of wireless transmission is strongly dependent on the intensity and phase noise attributes of the optically generated MMW signals, which heavily rely on the spectral purity and coherence of the optical sources generating them [2,5]. Thus, it requires optical sources with ultra-narrow optical linewidth and low relative intensity noise (RIN).

Various solutions have been proposed and demonstrated to obtain low phase and intensity noise in bulk / quantum-well (QW) based laser sources. This is done by either generating heterodyne signals from the same laser source with methods such as external modulation [6], and gain-switched optical frequency combs [4], or individual laser sources with schemes such as optical injection locking [7,8] and optical-phase-locked-loop (OPLL) [9]. Nevertheless, these methods are comparatively complex and not cost-effective. In contrast, the simplest approach of attaining high spectral purity is the design of dual-mode optical laser sources [10] with high coherence and operation in the C-band. This allows access to commercial telecommunication components to modulate, control, and manipulate the optical signals. However, most of the reported dual-mode lasers are based on either external cavities [11] or separate multi-section structures with independent bias control for each section [10,12]. Therefore, such optical laser sources either involve complex control circuitry or result in a relatively broad linewidth beat-note due to the uncorrelated phase noise of the two optical modes. Instead, the generation of concurrent dual-modes from a single cavity structure both sharing a single gain medium is very appealing due to device compactness, cost-effectiveness, high temperature stability and high spectral purity. Although simultaneous two wavelength lasing has been demonstrated in QW lasers using a single cavity structure [13–15], however, the two modes do not share exactly the same gain region and relatively high optical spectral linewidths were achieved. Thus, such sources most likely will results in a larger linewidth beat-note signals. On the other hand, quantum dot and quantum dash (QD) based lasers have been shown to have great potential as efficient optical beat sources due to the inherent characteristics of QD materials [16–18]. QD materials have superior characteristics due to their compactness, ultra-low threshold current density, ultra-fast carrier dynamics, improved temperature stability, and high spectral purity [19]. Owing to the inherent characteristics, QD materials based lasers are capable of achieving very narrow spectral linewidths and low RIN as compared to QWs [16]. These are the characteristics that make the QD based dual-wavelength lasers an attractive sources for generating MMW signals with high spectral purity and coherence.

In recent years, we have reported on InAs/InP QD Fabry-Perot (F-P) multi-wavelength lasers (QD-MWLs) emitting light over a large wavelength range covering the C- and L-bands with a channel spacing from 10 GHz to 1000 GHz and a total output power of up to 50 mW at room temperature [20–25]. Those QD-MWLs provide a simple multi-wavelength coherent comb source with the channel spacing determined by the laser cavity length. The unique properties of these QD lasers arise from the gain medium which is composed of millions of InAs semiconductor dots or dashes. Each QD acts like an isolated light source interacting independently of its neighbours and emits light at its own unique wavelength. In other words, the InAs QD gain medium is inhomogeneously broadened, unlike the uniform semiconductor layers in QW lasers that are deployed in telecommunications today. The combination of this inhomogeneous broadening and mode-locking in these F-P lasers results in a coherent multi-wavelength laser source where each channel is inherently stable with lower intensity noise and optical phase noise than comparable QW-based semiconductor lasers [16,26–29]. Thus, by making use of these unique properties of QD materials, we have designed, grown, fabricated and experimentally investigated a novel compact monolithic InAs/InP QD DW-DFB laser. These lasers work at room temperature and can cover an RF frequency range from 30 GHz to 300 GHz. Our experimental results show that this novel monolithic QD DW-DFB laser can achieve a very narrow optical linewidth down to 15.83 kHz and low RIN up to -158.3 dB/Hz in the frequency range from 10 MHz to 20 GHz. Using this laser we can generate high purity MMW signals between 46 GHz and 48 GHz with a record narrow linewidth of 26 kHz, which, as far as we know, is the best result reported so far for a single cavity-based free running dual-wavelength semiconductor laser with common cavity modes.

2. Quantum dash gain material growth, laser design and fabrication

The device studied in this paper was an InP-based p-n blocked buried heterostructure distributed feedback (BH DFB) laser. The undoped active region of the laser consisted of five stacked layers of InAs quantum dashes (QDs) with 10 nm In_0.816Ga_0.184As_0.392P_0.608 (1.15Q) barriers. This active layer was embedded in a 170 nm thick 1.15Q waveguiding core, providing both carrier and optical confinement. The grating was underneath the waveguiding core in the n-type InP cladding and was defined using e-beam lithography and wet etching. The growth of the dashes was performed in a manner very similar to that in [30], where to encourage the growth of dashes rather than dots, a thin layer of GaAs rather than GaP was grown before the InAs was deposited. Figure 1(a) shows a plan view scanning electron microscope (SEM) image of a typical quantum dash layer used for laser fabrication. Following the growth of the laser core, a mesa is formed by etching through the 1.15Q core and grating layer by employing a dielectric mask and a combination of dry and wet etching. A pnpn blocking layer structure was then selectively overgrown. The dielectric mask was removed and followed immediately by a final growth that consisted of p-type InP and contact layers. Figure 1(b) shows an SEM image of the output front cross section of a fully fabricated BH DFB laser. The inset in Fig. 1(b) is a lateral cross-section through the middle of the 2.0µm mesa showing a portion of the synthesized grating.

 

Fig. 1. SEM pictures: (a) top view of quantum dashes layer, (b) front cross section of the Buried heterostructure laser with inset showing lateral cross section of a portion of the synthesized grating at the middle of the mesa along the optical waveguide, (c) synthesized grating on the bottom of 5 quantum dashes layers.

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A novel synthesized aperiodic diffraction grating layer was designed to provide the distributed feedback such that two longitudinal modes would lase simultaneously. The desired reflection spectrum of the diffraction grating was obtained through optimization [31] of the transfer matrix formulation of the laser cavity. This method consists of combining multiple stop-bands with various amplitudes to create the desired spectrum. Optimization involved maximization of gain margin between the two desired modes and the Bragg side modes, as well as obtaining uniform optical field distribution along the laser cavity. Particular attention was paid to obtaining almost identical threshold gain for both desired modes. The result of the optimization is a non-uniform and aperiodic grating spanning total length of the laser cavity. Figure 1(c) shows a cross section of a portion of the synthesized grating underneath the active region with five stacked layers of quantum dashes, and Fig. 2 shows the resulting reflection spectrum of the synthesized grating with location of the two optical modes highlighted. It consists of three stop-bands with two central dips and a flat frequency response on the edges of the outer stop-bands. The gain margin between the two dominant modes and the next lowest threshold mode is calculated to be 45 cm^-1, and the threshold gain difference between the dominant modes is less than 10⁻³ cm^-1. The spacing between the modes was designed to be around 47 GHz. Both facets of the 1.8 mm long device had antireflective coating (AR).

 

Fig. 2. Reflection spectrum of a QD DW-DFB laser cavity.

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3. Experimental results and discussion

The laser chip was tested on a temperature controlled copper heat sink by applying continuous wave injection current. The output light of the laser was collected from the front facet through a collimator attached to a single mode polarization maintaining (PM) fiber. A two-stage PM optical isolator was incorporated after the collimator in order to avoid any back reflection into the laser cavity from the measurement system. All of the measurements were carried out at 18°C. Moreover, for the laser spectra measurements, an optical spectrum analyzer with a spectral resolution of 0.01 nm was used. The corresponding optical phase noise and RIN were measured with a commercial OEwaves OE4000 automated laser linewidth/phase noise measurement system and Agilent N4371A RIN measurement systems, respectively. A tunable filter (Santec OTF-970) was also used for filtering individual modes for analysis. For the MMW signal generation, the two optical modes were beat together on a Newport high speed photodetector (Model-1014) and the corresponding results were measured through a Keysight 50 GHz PXA signal analyzer (Model N9030A).

The characteristics of the laser and the generated MMW signals are analyzed in terms of spectral purity along with the dependency of the operating optical modes on the bias current applied to the device. We measured the light-current (L-I) characteristics, spectra, and behaviour of the optical modes as a function of injection current. Figure 3(a) shows the typical L-I characteristics of the device. Lasing starts at a threshold current of around 70 mA and the device shows stable linear behaviour in terms of output power as a function of injection current. Figure 3(c) shows the corresponding optical spectra of the device taken under different current biases ranging from 180 to 500 mA. Satellite modes are observed on either side of the two main modes due to four wave mixing (FWM) within the laser cavity. Figure 3(b) shows the power and emission wavelength of the four strongest lines. A redshift is observed with increasing drive current with the spacing between the two main modes increasing by 0.0158 nm when varying the current from 180 to 500 mA. The mode spacing corresponds to beat note frequencies between 46 and 48 GHz. Although the device exhibits stable dual-wavelength lasing in a large range of injection currents, the intensity of the two optical modes becomes equal around 360 mA with a wavelength spacing of 0.374 nm as depicted in the inset of Fig. 3(a) and Fig. 3(b). It should also be noted that at an injection current above 180 mA, the device starts exhibiting FWM signals at the shorter and longer wavelength sides of the two modes as shown in Fig. 3(c). These FWM signals stem from the mixing process that occurs within the laser cavity itself, which shows the spectral and temporal stability of the corresponding two optical modes. In addition, the FWM peaks follow the same trend, both in terms of intensity and wavelength, as that of the two optical modes as depicted in Fig. 3(b).

 

Fig. 3. Measured (a) L-I characteristics (inset shows the optical spectrum at 360 mA), (b) variation of the emitted dual-modes, FWM and their corresponding output powers as a function of bias current, and (c) spectra at different bias currents of the QD DW-DFB laser.

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The phase noise and RIN were measured experimentally for the two dominant optical modes. At a bias current of 360 mA, optical linewidths of 25.9 kHz and 29.4 kHz were measured for the two optical modes at λ₁ = 1539.522 nm and λ₂= 1539.896 nm, respectively. Their corresponding optical frequency noise spectra are shown in Fig. 4(a). Average RIN was measured as -150.8 dB/Hz for the first optical mode (λ₁) and -151.4 dB/Hz for the second mode (λ₂) over the frequency range from 10 MHz to 20 GHz. Figure 4(b) shows the corresponding RIN of the two optical modes. At a bias current of 465 mA, optical linewidth as narrow as 15.83 kHz and average RIN as low as -158.3 dB/Hz have been achieved.

 

Fig. 4. Measured (a) optical frequency noise spectra and (b) RIN spectra of the QD DW-DFB laser at a bias current of 360 mA.

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The MMW signals were generated between 46 and 48 GHz by heterodyne beating of the optical modes emitted from laser at different bias currents. Figures 5(a) and 5(b) show typical spectra of the corresponding beat note signals of 46.82639 GHz and 47.16556 GHz, respectively, which were generated at a bias current of 300 mA and 360 mA. For accurate measurement of the MMW spectral linewidths, the curves in Fig. 5 were smoothed (the black curves show smoothed results). The measured -3dB and -20dB spectral linewidths of the MMW signal at 46.82639 GHz are 26.1 kHz and 102.3 kHz, respectively. Similarly, for the MMW signal at 47.16556 GHz, the -3dB and -20dB spectral linewidths were measured to be 41.1 kHz and 257.1 kHz, respectively. To our knowledge, these are the best results reported so far for a common cavity based free running QD DW-DFB semiconductor laser operating in the 1550 nm wavelength range. These results are promising for applications as a compact MMW optical beat source in MMW Radar systems and heterodyne MMW communication systems for 5G and beyond mobile networks, particularly for achieving seamless fiber-wireless integrated fronthaul.

 

Fig. 5. Measured spectra (in red) with smoothing version of the results (in black) for -3 dB and -20 dB linewidth measurements of (a) 46.82639 GHz MMW signal at 300 mA (resolution bandwidth (RBW) = 51 kHz, video bandwidth (VBW) = 1 kHz) and (b) 47.16556 GHz MMW signal at 360 mA (RBW = 51 kHz, VBW = 510 Hz).

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4. Conclusion

We have developed and experimentally demonstrated a novel monolithic InAs/InP QD DW-DFB laser operating in the C-band for MMW signals generation. The device simultaneously generates two highly coherent optical modes with spectral linewidths as narrow as 15.83 kHz and average RIN down to -158.3 dB/Hz. Increased coherence of the two modes is due to high third order nonlinearity of the QD material resulting in the four wave mixing (FWM) phenomenon. Optical heterodyne beating of these two modes results in MMW signals between 46 and 48 GHz with extremely narrow linewidth, down to 26 kHz. These are the narrowest optical and beat note linewidths reported so far for a free running QD DW-DFB semiconductor laser operating in the 1550 nm band with common cavity modes. The results show that the demonstrated device is suitable for MMW applications, particularly Radar and high capacity MMW fiber-wireless integrated fronthaul for 5G and beyond.