Introduction

Photonic integrated circuits (PICs) based on InP have provided an effective solution to realize advanced functions at a system level with compact size. Other advantages include low power consumption, simpler coupling between elements and packaging resulting in higher reliability and lower cost [1]. Such PICs have been demonstrated using various techniques such as epitaxial regrowth [2], hybrid/heterogeneous integration of InP and Silicon [3], and band gap engineering methods such as quantum well intermixing [4,5] and selective area regrowth [6]. However, a disadvantage of these methods when compared with regrowth free monolithic integration is their fabrication complexity and duration [7].

As the demand for bandwidth-intensive services continues to grow, the increasing necessity of spectrally efficient solutions has led to the development of formats such as coherent wavelength division multiplexing (CoWDM) [8]. CoWDM requires the generation of a coherent optical comb and the demultiplexing and multiplexing of each subcarrier so that data can be placed on each individual subcarrier (Fig. 1). A low linewidth (~600 kHz) optical frequency comb has been demonstrated using the gain switching technique, via the monolithic integration of slotted Fabry-Perot (SFP) lasers into a three section device [9]. It was also shown that the lasers could be red shifted as far as 1584 nm by varying the bias. Selective amplification and filtering of wavelength injection locked SFP lasers has also been published, reporting the suppression of unwanted wavelengths in excess of 20 dB and a maximum gain achieved for a selected sub carrier was of the order of 18 dB [10]. This behavior could be applied to filter individual sub carriers of a comb while maintaining coherence. Therefore, the integration of these two pieces of work with a modulator and optical couplers/splitters [11,12] would produce a monolithic regrowth free CoWDM transmitter as in Fig. 1 . Electroabsorption modulators (EAMs) based on the quantum confined Stark effect (QCSE) [13] offer a compact solution for optical modulation, as their operating wavelength is comparable to that of a laser for a given epitaxy. The integration of a slotted laser structure with an EAM using e-beam lithography has previously been demonstrated in [14].

 

Fig. 1 Schematic of proposed regrowth free CoWDM transmitter based on SFP lasers and EAMs.

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In this paper, a single facet slotted Fabry-Perot laser monolithically integrated with an EAM is proposed and demonstrated. The absorption spectrum of the EAM was characterized using an off-chip tunable laser source (TLS), demonstrating the quantum confined Stark effect. The laser was successfully injection locked at various wavelengths up to 30 nm away from the free running peak wavelength. Modulating the EAM while the SFP laser was injection locked to 1590 nm produced an open eye diagram at 2.5 Gb/s. Fabrication costs are minimized as the device is UV lithography compatible and does not require epitaxial regrowth.

Device design and fabrication

A schematic of the device demonstrated in this paper is shown in Fig. 2. The single facet slotted laser [15] consists of a 600 µm long gain section which is enclosed by a 650 µm mirror section and a cleaved facet to create a lasing cavity. The mirror section has six uniformly separated slots (108 µm pitch) defining subsections of the laser cavity, providing optical feedback to the laser. The slots had a width and depth of 1 µm and 1.83 µm respectively. Wavelengths which are resonant with both the laser’s effective cavity length and these subsections are enhanced with other non resonant wavelengths being suppressed, inducing single mode operation through the Vernier effect [16]. The 150 µm EAM section curves to create a 7 degree facet which allows light to be coupled off-chip while minimizing reflections. A deep etched slot separates the mirror and EAM sections providing electrical isolation. Finally, a metal pad contacts the N semiconductor layers through an opening in the dielectric passivation creating a top side N metal.

 

Fig. 2 Schematic of the device proposed in this letter.

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The epitaxial structure used was commercially grown 1550 nm laser material on an N doped InP substrate. It contained five compressively strained AlInGaAs quantum wells with a total active region thickness of 0.4 µm. The device was fabricated using standard processing techniques, similar to [17]. The gain and mirror sections consist of 2.5 µm wide ridges defined by a shallow etch of 1.8 µm into the P layers. The 2.5 µm wide EAM section was produced by a deep etch of 2.4 µm that penetrated through the intrinsic region. This deep etch also exposed the N doped layers to enable the fabrication of the top side N metal contact. Both etches were achieved using a ${\text{Cl}}_2/{\text{CH}}_4/{\text{H}}_2$ inductively coupled plasma (ICP) etch. The device’s sidewalls were passivated with a 300 nm thick layer of silicon dioxide grown by plasma enhance chemical vapour deposition (PECVD). Titanium and gold were deposited by standard lift-off lithography and e-beam evaporation to form the P and N contacts on the surface of the chip. A 360-degree rotational tool was used to ensure the metal ran continuously up the sidewall, reducing potential metal breaks. The wafer was then chemically thinned with a bromine methanol solution to enable high quality facets to be cleaved. Images of the device during fabrication are shown in Fig. 3.

 

Fig. 3 Scanning electron microscope (SEM) image of (a) a shallow slot from the mirror section and (b) the deep etch electroabsorption modulator and angled facet.

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Results and discussion

The gain and mirror sections of the laser were power by DC probes while the EAM and the topside N Metal were contacted using the high speed GS probe. The N metal acted as a common ground for the entire device. Short focus lensed fibers were positioned close to the outputs of the devices to enable coupling to the facets. Initial testing of the device focused on characterizing the absorption spectrum of the EAM. This was done by coupling light directly into the EAM section via the angled facet using an external TLS. The wavelength of the TLS was swept from 1540 nm to 1610 nm in 10 nm increments. The corresponding photocurrent was recorded from the EAM for varying applied voltage and is plotted in Fig. 4. The applied voltage causes a reduction of the band gap energy. This reduction results in a red shift of the absorption spectra. Therefore, longer wavelengths which are not being absorbed at 0V being to experience attenuation at higher biases. This field induced absorption of longer wavelengths demonstrates the quantum confined Stark effect. This information is crucial for the integration of a single mode light source with the EAM as it indicates that the laser needs to operate at a wavelength of 1580 nm or longer to properly utilize the QCSE. Encouragingly, this wavelength is comparable to peak wavelength of the lasers of the previously mention comb source [9], which used an identical epitaxy design.

 

Fig. 4 EAM absorption for an off-chip tuneable laser source.

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Both laser sections were biased to just above threshold (28 mA) with the EAM section lightly biased (5 mA) to minimize absorption losses. The output of the laser was coupled from the angled facet into an optical spectrum analyser (OSA). The resulting optical spectrum was then converted from the wavelength domain to the spatial domain plot in Fig. 5 by taking its Fourier Transform (assuming the group index to be 3.5). A large peak in Fig. 5 is clearly visible at 1290 µm (A), corresponding to the length of the gain and mirror section of the laser. Peaks representing the sub cavities created by the mirrors slot are also visible (B). This analysis confirms that the EAM section is not part of the lasing cavity and therefore, should not influence the lasers optical spectrum when modulated. The free spectral range (FSR) of the laser was calculated to be 3.17 nm [16]. This FSR is dictated by the separation of the slots in the mirror section and therefore can be manipulated as required.

 

Fig. 5 Fourier transform analysis of laser’s optical spectrum near threshold.

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Wavelength injection locking of the single facet laser was achieved by coupling light from the TLS directly into the gain section of the laser using the 90 degree facet. The behavior of the laser was monitored by recording the optical output from the angled facet. Figure 6 shows the optical spectrum of the free running laser and injection locked in approximately 5nm increments from 1570nm to 1595nm. This result demonstrates that the slotted laser can be red shifted sufficiently by means of injection locking, to operate with an EAM. The maximum measured optical power from the laser was <1mW, however, this is due to the limitations of the coupling efficiency with single mode fiber in our on-chip test setup. The fiber coupled output power could be improved by utilizing a mode adapter or an optimized lensed fiber [18].

 

Fig. 6 Optical spectra of free running and injection locked laser with external tuneable laser source.

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As in Fig. 6, the SFP laser was externally injection locked to 1590 nm using an external TLS. A pseudorandom binary sequence (PRBS) source provided a 2.5Gb/s signal with a pattern length of $2^7$-1 and a tone spacing of 78.7MHz. This sequence was passed through an RF amplifier to increase the peak to peak voltage of the signal to 1V. Finally, this amplified RF signal was combined with a DC bias of −1V before being applied to the EAM via a bias tee and high speed GS probes. The output signal from the device was boosted by an L band Erbium doped fiber amplifier (EDFA) to compensate for low optical power due to coupling losses. The quality of the signal was then analyzed with a 10 GHz photodiode and a digital communications analyzer (DCA). Figure 7 displays the observed 2.5 Gb/s eye diagram. The x and y scales of the diagram are 7.4 mV/div and 100.0 ps/div respectively. The eye is clearly open which confirms that the device is working as intended. Furthermore, this result demonstrates that such a device could potentially be applied in the CoDWDM transmitter described in Fig. 1. A DC measurement of the optical output power of the device with the laser injection locked at 1590 nm and EAM biased at −0.5 V and −1.5 V respectively (corresponding to the maximum and minimum values of the applied RF signal) gave an extinction of approximately 8.6 dB.

 

Fig. 7 2.5 Gb/s eye diagram from injection locked single facet slotted laser integrated with an electroabsorption modulator.

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To quantify the limitations of this device, a vector network analyzer (VNA) was used to measure the electro optic 3 dB bandwidth of the EAM. For this measurement, the SFP Laser remained externally injection locked at 1590 nm and the EAM again received a DC bias of −1V. Figure 8 below shows the electro-optic S21 response of the EAM between 0.1 and 10 GHz. The 3 dB bandwidth was calculated to be approximately 5.25 GHz. This low bandwidth can be attributed to the use of an N doped substrate and the fact that the EAMs contact pad is separated from the N doped semiconductor layers by only 300 nm of dielectric which creates a significant parasitic capacitance. The high speed performance of the EAM could be improved by using a thicker dielectric layer or by using semi-insulating (SI) substrates and more advance high speed EAM designs such as is demonstrated in [19].

 

Fig. 8 Electro-optic S21 measurement of electroabsorption modulator.

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Conclusion

This work in this paper demonstrates a single facet slotted laser injection locked up to 30 nm away from the main free running lasing mode. This compliments the optical absorption characteristics of the EAM which show that for this particular epitaxy, a wavelength of 1580nm or larger is required to properly take advantage of the quantum confined Stark effect. Utilizing these results, an injection locked SFP laser has been integrated with an EAM to produce a 2.5 GB/s eye diagram. The 3 dB electro optic bandwidth of the EAM was measured as approximately 5.25 GHz. Therefore, we expect that the bit rate of the eye can be significantly increased with the use of more advanced epitaxy and EAM designs. A proposed application for this work is to combine the on chip comb generation demonstrated in [9], the comb filtering capabilities of slotted FP lasers from [10] and the integration of a slotted FP laser and EAM from this paper to create a regrowth free monolithic CoWDM transmitter.