We present the first demonstration of an InAs/InP Quantum Dash based single-section frequency comb generator designed for use in photonic integrated circuits (PICs). The laser cavity is closed using a specifically designed Bragg reflector without compromising the mode-locking performance of the self pulsating laser. This enables the integration of single-section mode-locked laser in photonic integrated circuits as on-chip frequency comb generators. We also investigate the relations between cavity modes in such a device and demonstrate how the dispersion of the complex mode frequencies induced by the Bragg grating implies a violation of the equi-distance between the adjacent mode frequencies and, therefore, forbids the locking of the modes in a classical Bragg Device. Finally we integrate such a Bragg Mirror based laser with Semiconductor Optical Amplifier (SOA) to demonstrate the monolithic integration of QDash based low phase noise sources in PICs.
© 2014 Optical Society of America
InAs Quantum dash (QDash) based lasers grown on commercially favoured InP(100) substrate have been of significant interest in recent years. This material system has been used to perform several demonstrations for Directly modulated lasers [1, 2] as well as for Mode locked lasers (MLLs). MLLs based on QDashes have been studied extensively both as frequency comb generators  and as sub-pico-second pulsed laser sources. These QDash MLL have been investigated in both the two-section [4, 5] and single section [6, 7] configurations. Of particular interest are single section devices, which exhibit self-mode locking and self-pulsation. In QDash lasers the phase relations between the optical modes allows to achieve typical MLL pulsations after propagation of the emitted light through a properly chosen length of standard single mode fibre.
It is therefore of great interest to monolithically integrate these frequency comb generators in PICs to fully exploit the unique performance of these lasers . As the QDash lasers exhibits a wide spectral envelope, in the form of a frequency comb, the prime interest is to enable the integration with other devices on the indium phosphide platform and a possibility to set the channel spacing with lithographic precision. SOAs integrated to quantum well based MLL have been previously demonstrated by Akbar et al.  and by Sato et al , but the Radio-Frequency (RF) linewidth of the Quantum Well based MLs remains in the mega-Hertz range, making them maladapted for many applications. On the other hand singlesection QDash Fabry-Perot (FP) lasers exhibit RF linewidths as low as 10 kHz, making them interesting for low timing jitter applications.
In this paper we present a detailed study of an effective Bragg grating (BG) design that can be efficiently used to close the cavity without compromising the mode locking performance of the QDash laser. This approach hence maintains a kilo-Hertz order RF-line-width intrinsic to QDash Material system. As a demonstration of integration, we include on the laser bar a QDash-SOA. We also report for the first time, to our knowledge, on the integration of QDash based devices on InP and we demonstrate the on-chip generation and amplification of optical frequency combs for telecommunication applications generated by single-section QDash based ML laser.
2. InAs/InP quantum dash material
The accurate control over the material quality and the know-how in the growth of the InAs/InP quantum dashes has allowed numerous demonstrations for MLL based on this material system. For the present study the laser active region is composed of 6 InAs quantum dash layers embedded in InGaAsP barriers in dash-in-a-barrier (DBAR) design . Buried ridge stripe (BRS) technology has been used for the fabrication of laser diodes. In order to assess the quality of the epitaxial structure for the fabrication of MLL, FP laser diodes have been processed. As shown in Fig. 1, threshold current as low as 20 mA and optical power output as large as 40mW can be achieved at 25°C for a 1000 μm long FP device having a ridge widths of 1.5 μm.
A FP laser from such material also leads to a narrow RF line-widths of the order of tens of kilohertz. Fast photo-detection of the signal emitted by the device demonstrates effective mode locking for operating temperatures up to 90°C, highlighting the potential of QDash lasers for uncooled operation. RF line-widths down to 35 kHz and emission spectrum FWHM up to 12 nm (∼1.5 THz) were measured for a large range of temperatures on FP lasers of length 1000 μm and ridge width 1.5 μm, as shown in Fig. 2. These results demonstrate the robustness of frequency combs generated by Quantum Dash material.
3. Distributed Bragg reflector design and theory
In this section we provide a theoretical analysis of the Bragg reflector design and identify the parameters of a Bragg reflector suitable to close the cavity of a FP Laser, maintaining the envelope in the orders of 10nm to 12nm. The specific requirement for such a Bragg reflector is a large passband to allow the entire FP spectrum to fit in, as opposed to conventional for the DBR lasers, where it is preferable to have a very narrow passband allowing emission of a single wavelength.
The temporal-spatial evolution of the slowly varying complex amplitudes of the counter-propagating optical fields E+(z, t) and E−(z, t) within the uniform Bragg grating can be modelled by the travelling wave (TW) equations :Fig. 3. Let us assume a non vanishing field incoming into the grating through its left side (Ê+(0, ω) ≠ 0), as well as a vanishing field reflection (R1 = 0) and, therefore, a vanishing backward propagating field at the right edge of the grating (Ê−(LBG, ω) = 0). Then the Bragg grating induced wavelength (or frequency) dependent field amplitude and intensity reflections and RBG(ω) = |rBG(ω)|2 are determined by
Equivalent expressions can be derived using the coupled mode formalism of Erdogan et al .
Using the above definition for the Bragg mirror, we modelled different gratings while maintaining the same peak reflectivity for each of the grating. Figure 4 shows the modeled reflectivity spectra of the designed Bragg gratings.
With increase in coupling coefficient for the same length of the grating, both the reflectivity and the pass band increases, while increasing the length of the grating produces a minor increase in bandwidth while largely increasing the reflectivity. Thus, in order to emulate an as-cleaved facet, allowing the full mode-locked spectrum to pass through the Bragg grating, we use a grating with large coupling coefficient κ and a small length LBG.
4. Device fabrication and influence on mode locking characteristics
Devices were fabricated in buried ridge strip (BRS) geometry to qualify the Bragg grating, and also the influence of internal filtering on mode locking characteristics due to the Bragg mirrors. Three different types of grating-mirror lasers were fabricated with the pass bands as shown in Fig. 4. The desired coupling coefficient of each Bragg grating was achieved by controlling the etch depth of the grating. These gratings are etched on the ridge such that the effective gain section length is 1000 μm, giving an expected free spectral range of around 40 GHz. The fabricated structure had a gain section of 500 μm, and a Bragg section of 500 μm. The gain and Bragg sections have separate electrodes, which are connected together using a wire bonding. Thus electrical injection occurs in both gain and Bragg sections.
The gratings with low coupling coefficient (κ = 40cm−1) were fabricated with smaller etch depth than those with high coupling coefficient (κ = 400cm−1), for which a deep etch process was used. It is observed that increasing the passband of the Bragg grating produces a narrowing of the minimum RF linewidth achievable for the device. Figure 5 presents a comparison of the different Bragg gratings with the corresponding RF line-widths. Line-width values down to 40 kHz can be achieved for the laser with a passband of 10 nm which are comparable to those of as-cleaved FP lasers.
It can be noted that the deep etch used for high coupling coefficient modifies the index contrast and hence for the same pitch of the grating the laser spectrum shifts towards longer wavelengths. It appears that for self-mode locked single section devices, in order to obtain a narrow RF-line-width and consequently low timing jitter pulse generation , it is necessary to have a certain number of longitudinal modes propagating inside the cavity. In section 5 we show that this fact can be attributed to the influence of the intrinsic phase modifications due to Bragg gratings.
For the laser with the largest pass band, the dependence of repetition frequency and RF line-width is studied as a function of injection current. Figure 6(a) shows a mapping of the RF spectrum with the laser bias current and the RF line-width for a current value of 300 mA.
The RF line-width is observed to assume lower values for high injection currents, with discrete regions of instability determined by the laser dynamics. Regions of effective mode locking are highlighted in the mapping in Fig. 6(a). Effective mode-locking with RF line-widths on the order of 100 kHz and below is observed for broad current ranges of 20 to 50 mA around the current values of 300 and 450 mA. The fluctuation in the RF linewidth around these regions are related to dynamics of the MLLs, which had been studied in details by e.g. Kefelian et al . A comparison of a FP laser of 1000 μm length is presented in Fig. 6(c).
5. Influence of Bragg grating design on intrinsic phase of optical modes
It is observed from the results on the various designs of the Bragg gratings that the gratings with the low passband would result in a laser with very broad radio-frequency linewidth resulting in no auto-pulsations or very broad pulses. This can be attributed to the modification of the optical modes due to the Bragg grating. To evaluate an impact of the Bragg gratings on optical modes we perform a mathematical study of the TW equations (1) governing dynamics of the optical fields in a whole ML Bragg laser represented schematically in Fig. 3. The TW equations (1) are supplemented by the reflecting boundary conditions at the facets z = −LG and z = LBG of the laser:
Like in discussions of Section 3 we ignore the spontaneous emission and gain dispersion 𝒟 in (1), assuming that the gain spectra within the wavelength range of our interest is flat, whereas the main contribution to the gain dispersion comes from the BG part of the laser.
Let us rewrite the TW equations (1) in the operator form(7) gives rise to the spectral problem (9). Complex vector-eigenfunctions Θ(β, z) define spatial distribution of the optical modes, whereas real and imaginary parts of the complex eigenvalues Ω represent the main contribution to the optical frequency and the damping of the optical mode, respectively . That is, the amplitude fk(t) of the k-th mode evolves according to 16, 17].
In general, the complex propagation factor β (z, t) in the QDot or QDash lasers operating at the ground state (GS) depends on the GS occupation probability . For ML lasers with a nearly constant emission intensity discussed in this paper this parameter should be nearly constant in time, and, therefore, should have a rather uniform distribution within each of the gain and the Bragg grating parts of the device. For simplicity, in the following analysis we assume that within all laser device β remains independent on carriers, i.e., β ≡ −iα/2, where α is the scattering losses of the field within the laser. This assumption does not allow to simulate the laser dynamics, but admits a proper description of the relations between the complex mode frequencies Ω(β). In the case of the FP laser (κ ≡ 0, LBG = 0, R1 = 0.6) one can easily solve Eq. (9) and find an infinite number of complex frequencies Ω, all of which have the same dampingFigs. 7(a) and 7(b). Such configuration of the mode frequencies is one of the decisive factors for the ML pulsations, since the complex mode amplitudes fk mainly evolve according to
The introduction of the BG changes the relative positions of the complex mode frequencies Ω. For κBG = 40 cm−1 and LBG = 250 μm only a few modes located within the ∼ 300GHz wide stop-band have similar thresholds, whereas all other modes are strongly damped (green triangles in Fig. 7(a)). A typical performance of such distributed Bragg reflector (DBR) laser is cw operation at the maximal gain mode, or the mode-beating type pulsations involving a couple of modes . No good quality ML pulsations can be expected (see also Figs. 5(a) and 5(d)).
In the cases of κBG = 200 cm−1, LBG = 50 μm (diamonds) and κBG = 400 cm−1, LBG = 25 μm (squares) the damping of 20 or even more modes within the stop band is not very different. Actually, a similar modal gain dispersion can be implied by the dispersion operator 𝒟 (omitted in this case), which, nevertheless, does not destroy ML in the lasers with saturable absorber . A more crucial for ML effect of the BG is represented in Fig. 7(b). The implementation of the BG violates the equidistance of the mode frequencies. In the case of the laser with κBG = 400 cm−1 (squares) the frequency separations of ∼ 20 adjacent modes shown in Fig. 7 vary between 42.28 and 42.32 GHz. On the other hand, these mode frequency separations in the laser with κBG = 200 cm−1 (diamonds) vary in almost by an order larger frequency range between 42.7 and 43 GHZ.
To our opinion, this strong variation of the mode separations is the main effect which drastically increases the RF-line-width as observed in experiments: cf. panels (e) and (f) of Fig. 5.
6. Integration of Bragg reflector laser with a semiconductor optical amplifier
To exploit the Bragg mirror approach for integration we fabricated the devices integrated monolithically to a quantum dash based semiconductor optical amplifier (SOA). Figure 8 shows a micrograph picture of the device in which the first two sections on the left act as gain section, followed by an SOA with tilted taper. The length of the gain section is 1000 μm and that of the tapered SOA section of 1000 μm. The tapered facet of the SOA is anti-reflection coated.
The Light Current characteristics of the Bragg Mirrored Laser can also be seen in Fig. 8, where figure on the left show pure characteristics of the Laser, while the figure on the right indicated the amplification produced due to the SOA.
The radio frequency mapping was repeated after the amplification of the comb by the SOA, and it is observed that the SOA does not induce significant changes to the locking performance of the laser. This has also been observed by Akbar et al.  on conventional quantum well material. A similar RF line width is obtained after amplification from the SOA. The narrowest RF line width after amplification of comb is found to be ∼33 kHz. The RF mapping for constant current of 300 mA in the gain sections and varying current in the SOA and the RF linewidth for an SOA current of 200 mA is shown in Fig. 9.
The narrowest RF line-width with and without bias current for the SOA section are found to be comparable. However, current injection in the SOA section produces a change in the regions of effective mode-locking, with respect to laser operation in absence of bias on the SOA. This effect is attributed to the interaction of the laser emission with the amplified spontaneous emission coming from the SOA, in addition to the device heating due to current injection in the SOA. Effective mode-locking regions for current ranges in excess of 50 mA can still be found. The narrow RF line-widths is accompanied by self pulsation with puse duration down to 1.4 ps, measured by intensity autocorrelation, after chirp compensation with 120 m of SMF. Thanks to the mode shape converter of the SOA, an effective gain of about +10 dB can be obtained in the optical power coupled in the fiber. To the best of our knowledge, this is the first demonstration of integration of a low phase noise comb source with SOA based on quantum dashes on InP. This opens the way for integration of QDash lasers with e.g. a modulator section for Radio-over-Fibre applications.
6.1. Frequency comb modulation
It is desirable to modulate the mode locked lasers for certain applications, like Radio over Fibre (RoF). In RoF the mode beating frequency is used as a carrier wave thus eliminating the need of an external RF source. However it is still necessary to modulate the light signal with the data, which is performed using an external Mach Zehnder modulator or by directly modulating the laser. The direct modulation scheme is favourable due to its compactness but it had been demonstrated that the direct modulation of the gain section produces a perturbation of the RF-line-width . The monolithic integration of a modulator section with the Bragg laser would solve the issue. The device as shown in Fig. 8 can be configured to modulate the SOA to obtain a modulated frequency comb. Up to 5 Gbps on-off keying modulation was demonstrated on the present device, as shown in Fig. 10.
Such BRS based SOA is however not the ideal candidate for the modulation operation but it demonstrates the potential of the Bragg section for the PICs. Thus, if used with a short modulator section, this approach could provide efficient modulation without producing any significant changes in the mode-locking characteristics of the laser.
We report on the first demonstration of a Bragg based QDash-MLL which opens the way for integration of QDash based frequency combs generators on InP photonic integrated circuits. A specific Bragg mirror design is presented which allows to close the cavity without compromising the ML performance. Based on a detailed travelling-wave model it is found that the difference of mode-separation due to dispersion of the grating can be minimized for gratings with high coupling coefficient. This allows modes to phase lock just as they do in a single section FP device. The approach of Bragg mirrors for closing the cavity is of particular interest as it allows integration of such lasers on photonic integrated circuits and in addition provide with a possibility to set the channel spacing and repetition frequency with lithographic precisions. We have integrated this Bragg-ML laser with a semiconductor optical amplifier as a demonstration of the potential of this approach for integration. The Bragg mirror can also be used to compensate the laser intra-cavity dispersion as presented by Sato et al.  and Strain et al. , but this would need detailed study on the material dispersion property of QDash material and also consequent change in the design of the Bragg mirror according to the gain spectrum of QDashes.
The authors would like to acknowledge the support of EU FP7 ITN PROPHET, Grant No. 264687 and Agence Nationale de la Recherche, France for TELDOT project. M. Radziunas has been also supported by DFG Research Center Matheon.
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